Australian teenager Ben Simmons is listed at 6 feet 9 1/2 inches (207 centimetres) and 229 pounds (104 kilograms). By the time you read this, his weight could be 110 kilograms or more: this is a kid who is bulking up.

At 19, Simmons has just been voted the best player in American high school basketball. Next June he will be eligible for the 2016 National Basketball Association (NBA) draft. Many believe that he will be the first player picked, thus earning a starting salary of $US5 million, but for the time being, on the cusp of a superstar life and career, he is an amateur and must abide by National College Athletic Association (NCAA) rules. The free sneakers, shorts, vests and socks that players receive at the academy are the nearest thing to a sponsorship that he can accept.

Downstairs in the lounge at the Westin Hotel, where the academy is based, a DJ spins hip-hop at club volume. Masseurs are on hand, there is a barber shop, plus ping pong tables, PlayStations and a fairground game with miniature basketballs that the players ignore. Stepping into the hotel's lift, chest-high to the young men around me, I feel about eight years old. Except they are in high school and I'm old enough to be their father.

A TV is showing Sole Man, a documentary about Sonny Vaccaro, the US marketing genius who turned basketball into a multibillion-dollar industry by giving away sneakers. The film is an indictment of the "student-athlete" system, in which coaches, colleges and corporations get rich off the backs of unpaid players.

Either it just happens to be showing, or one of the organisers has a dark sense of humour. Because there in the middle of the room, lit up like holy relics, are the sneakers, all named after famous players: the LeBron Soldier 9 (after Cleveland Cavaliers player LeBron James), the KD 8 (after Oklahoma City Thunder forward Kevin Durant) and the Kyrie 1 (after Cleveland Cavaliers point guard Kyrie Irving, who was born in Melbourne).The players receive a free pair of each but, for now, Simmons' will stay in the box. "I'm kind of particular about what I wear," he says. "On the court I definitely wear LeBrons. I wouldn't wear anything else."

Simmons doesn't barrack for an NBA team. It wouldn't be wise, seeing as any one of 30 franchises could draft him, but mostly it's because he's a LeBron fan. "I know LeBron," he says with a laugh, as if he can't quite believe it. "He's like a mentor to me. We call every now and again. I'll text him if there's something I need to know." They met on court at the LeBron James Skills Academy in Las Vegas, in July 2013. "I was going at him and he was going at me the whole time. After that, I guess he had a respect for me." James, 30, recently took his team to the NBA finals. When Simmons faces him next, the pace and stakes will be higher.

The LeBron camp and the Nike Academy are just two of the many events designed to foster brand loyalty, develop young talent and promote the sport of basketball. Adidas, Reebok and Under Armour all have their own versions. Simmons has been attending the camps since the summer of 2012, when he turned up with his dad, Dave, a former professional player, at the Pangos All-American Camp in California as a scrawny 15-year-old with freakish skills – and left with high schools clamouring for his signature.

After a long chat with his parents, Simmons opted for Montverde Academy, a boarding school near Orlando, Florida, which meant leaving his family behind in Melbourne, where he had attended Box Hill Senior Secondary College. In his first game, a player from the opposing team went to the baseline and dunked on him. "It was like, 'Wow, the speed of the game is different to what I'm used to.' You could see that expression on his face," recalls Montverde coach Rae Miller.

That season, Simmons made crucial contributions from the bench as his team went on to win the US high-school national championship. They won again the following season, with Simmons starting at power forward. This April, he led the team to a third straight title and was named the tournament's Most Valuable Player. In August 2013, Simmons made his debut for Australia's national team, the Boomers, against New Zealand, becoming the youngest player to represent Australia. Coming off the bench, he grabbed a steal, ran up court and dunked two-handed, confusing the television announcer: "David Simmons with the pressure ..."

At the Nike camp, I ask basketball experts whether his game reminds them of a particular player. A few say Magic Johnson or Tracy McGrady – big men with point guard skills – but the most common answer is his hero, King James. "Everybody compares him to LeBron," says Simmons' godfather, David Patrick, who has just started coaching him at Louisiana State University. He laughs at the audacity of it. "I know that's a high bar, but there's not many guys that tall that can handle the ball."

When Simmons made his choice of college public, four-time NBA champion Shaquille O'Neal, the university's most famous alumnus, tweeted: "LSU just signed the best player in the world. Check him out."

When Julie Simmons found out she was pregnant in November 1995, she was overjoyed – and surprised. She and Dave, her second husband, had been trying for a long time to conceive a sibling for their daughter, Olivia. Julie had four children from her previous marriage. This was her last chance.

The contractions started on July 20, 1996, midway through the opening ceremony of the Olympic Games in Atlanta. In her room at Melbourne's St Vincent's hospital, Dave switched on the telly to watch his Melbourne Tigers teammates Andrew Gaze and Mark Bradtke march past in the parade. A doctor was supposed to be examining Julie, but his attention drifted, too.

"Nothing's happening. We almost had forgotten she was there," Dave recalls. "They had a big sign [at the ceremony] saying, 'How y'all doin'?' and Julie looked up and screamed, 'I'll tell you how I'm doing. I'm trying to have a baby.' " Benjamin David Simmons weighed 3.5 kilograms. His parents took the timing of his arrival as a sign, although of what they weren't sure.

Dave Simmons was a legend in Australia's National Basketball League who, together with fellow American import Dave Colbert – they called themselves The Double Ds – transformed a losing culture at the Melbourne Tigers to the team's first championship in 1993. "I was very physical. Finesse wasn't my thing," Dave admits.

As a kid in the South Bronx, Dave Simmons played centre for his high school team. Rapper Easy A.D., of the Cold Crush Brothers, was his point guard. After graduating, he knocked around leagues in Costa Rica, Colombia and Venezuela until he got a call from his agent: an Austrian team were in town and there might be a job for him.

He got himself to the hotel, wondering if they spoke English, and met not an Austrian but then Melbourne Tigers coach Lindsay Gaze (father of renowned player Andrew Gaze). "Lindsay Gaze walks up to me and goes, 'G'day mate.' He's got five coins, showing me the offence he's going to run for that night," Simmons recalls. "I'm thinking, 'This guy's crazy', but eight years down the track [we were] still running that offence and it worked out all right." In Melbourne, Dave met Julie at a gym.

He took on her four sporty children – Melissa, Emily, Liam and Sean – as if they were his own, and together they had Olivia and Ben. Ben, the youngest by five years, tagged along to basketball, rowing, AFL and cricket. "We were like, 'Throw him a ball, see if he can dribble', from the time Ben could take his dummy out of his mouth and shoot it like that into the sink," says Dave, miming the shot.

By the age of four, Ben was competing in basketball against seven-year-olds. At seven, he played for the under-12s. Allen Iverson was the NBA's reigning shooting guard and, after watching the weekly highlights program, Sean and Liam taught their little brother Iverson's signature crossover moves. "The next game we went to ... you're talking about a seven-year-old, and here he is, putting it through his legs and occasionally getting it around his back," remembers Liam.

The night before Nike camp, the NBA 2015 draft, attended by 10,000 people, was held at the Barclays Centre in Brooklyn. The draft system is convoluted, but essentially the teams at the bottom of the league get to pick first, to even things out. With the second pick, the Los Angeles Lakers selected D'Angelo Russell, who played with Ben Simmons at Montverde from 2012 to 2014. "We used to sit in the dorm rooms and talk about how the draft is so far away for him, and then for me," muses Simmons. "It seems like it was yesterday."

In his junior year at high school, Simmons announced he would be going to Louisiana State, disappointing the coaches from Duke, Kansas, Florida, Miami, Kentucky, Michigan and UCLA that had come to watch him practice. In May, LSU, not a traditional basketball power, launched an ad campaign featuring the torso of a player clearly meant to be Simmons, with the tagline: "He's coming." To defuse the outrage that greeted this brazen marketing of an amateur athlete, the university pointed out that Simmons had signed off on the ads. "Ben's excited. It makes him look like Jesus," his half-sister, Emily Tribe-Simmons, tells me.

Tribe-Simmons moved to the US on a rowing scholarship and married Michael Bush, a running back in the National Football League. She works in sports marketing and, as Ben is officially an amateur and cannot have an agent or a publicist, is handling media for him. One of the first things she tells me is, "If you go out for dinner with Ben, he has to pay for his own meal."

Simmons can be suspended for the tiniest infraction. The issues surrounding student athletes have been a source of controversy. The NCAA recently lost a court case brought by a former college basketball player, Ed O'Bannon, who objected to the organisation making money off his likeness. It is appealing the decision and has since tweaked the rules, introducing a "living expenses" payment of $US2000 a year to players.

Last year, the South-Eastern Conference – in which Simmons' team at LSU competes – alone generated $US1 billion in profit, if you combine football and basketball receipts, merchandising and television revenue. Top coaches are paid seven-figure salaries. At nike academy, the nba scouts won't go on the record, but privately acknowledge that Simmons deserves his ranking. "He could have gone this year and been drafted top three. He's a basketball player," says one, tapping the side of his head. "Know what I mean? He makes everyone around him better."

This is often attributed to his upbringing in Australian basketball, with its pass-first mentality. In the US, top players are expected to impose their will on the game. Time and again, I hear Simmons has "a high basketball IQ" and is "a man among boys". The latter may have been true in high school, but it isn't here, up against college athletes. From the first two on two, the speed and intensity is frightening. Only a handful of these players will make it to the NBA, but they are all huge, strong, skilled and determined.

One of Simmons' recent tweets showed a drawer full of protein shakes: "Stocked up! Gain gain gain." Liam and Sean, who often go months without seeing their younger brother, both say they've been shocked by the size of him. "His body type has changed," says Sean. "It's almost an unfair advantage, but it's a credit to him, because that's what's been driving him to his weights sessions at 6am. I remember at one point he was eating nori, just seaweed, because his trainer told him to. That's the kind of commitment he has."

Adds Liam, who played college basketball himself and is now assistant coach at Southwest Baptist University in Missouri:"I was getting phone calls from Australia, 'Man, the other night he did this and it was unbelievable.' It always came down to how hard he wanted to work. Are you willing to do the little things that no one else is willing to do? You can't have your brother want it more than you. You can't have your dad want it more than you."

There are no guarantees it will pay off. This time last year, Cliff Alexander was one of the top five high-school players in the US, but after an underwhelming stint at the University of Kansas, no NBA team wanted him. To date, Simmons' one big disappointment was being cut from the Boomers squad for the World Championships last year. "I had to go back to school anyway, so I looked at it as a positive," he says, but it's clear that it still smarts. "At the same time I was kind of mad. I knew I should have been on the team."

On the last day of Nike camp, lebron james joins the college players for a yoga session in the morning, and in the afternoon, Simmons goes up against NBA All-Star James Harden. The hangar at Santa Monica Airport, which has been laid with four courts, is packed with family members, scouts, reporters and basketball grandees. Power-broker Dinos Trigonis tweets that Simmons is "more than holding his own" against Harden.

Dave Simmons takes it all in from the stands. He always comes, and if he can't, he makes sure that Emily or Liam or Sean can. "It looks like Nike already have Ben sewn up," I offer, "seeing as he only wears LeBrons." Dad chews on this for a few seconds. "Well, he loves the LeBron shoe, but if we're fortunate enough to get to the next step, they'll make a shoe for him," he says. "It'll be the Ben Simmons."