Unable to speak, he withdrew from people at school and, worse, from his family.
In March 2003, Miller was introduced to Jeff Hancock, a one-on-one therapist for people with autism, and things started to feel a little more manageable.
Hancock was hired to be John's companion. The two would go people-watching at the mall in Stuart, Florida, on the weekends. Hancock chatted throughout their excursions, even though John couldn't talk back.
Their relationship evolved when Hancock introduced John to the Special Olympics in Martin County. Hancock, a longtime volunteer for the organization, and Miller decided John should get involved in sports after they saw a rare sight: Whenever John was running or being active, he was smiling.
Up until that point, Miller said, it was virtually impossible to get John involved with sports because of his condition.
"There were very few people who knew how to handle a disability as severe as John's," she said. "Even teachers and professionals were unable to reach him."
One area of difficulty was communication. John couldn't speak and was mainly unresponsive to people.
And people distanced themselves when they saw the teen injure himself. John would bite his arms and hands and even hit himself with powerful blows to the head, Miller said.
As John got older, his behavioral issues intensified. But while teachers and peers drew away, Hancock stood closer by his side.
When John turned 12, Hancock introduced him to track at the Special Olympics. For the first three years, the pair just observed the sport by watching practices and events. Hancock worked daily with John, showing him everything, including where to sit, stand and walk, so he would understand the flow of the events.
Eventually, the lessons clicked, and John was off doing 50-meter sprints. John was racing because he loved to run, not because he wanted to compete, Hancock explained.
"Winning is not in his vocabulary. We put him in smaller races so that he could finish," Hancock said. Finishing the races was a way to boost John's confidence. "He learned that when he finished the race he could see his mom, and get a drink, and get hugs."
But as John got older, he got stronger and faster. At 16, he was jumping hurdles and doing 800-meter races. Miller started seeing her son as a serious athlete.
Off the field, John was closed off to the world, but on the track he shined.
"John's head will be down a lot when he is trying to have social interactions. But when he is running, his form is perfect," she said.
John even verbalizes on the field, joyously yelling. "It sounds like he is happy," Miller said.
In May 2014, John's training brought him to the Special Olympics State Summer Games in Orlando. The 18-year-old was competing in the 800 meters and the 110-meter hurdles.
As the 800 was setting up, Miller remembered, she pressed her face against the fence, anxiously waiting for John to compete. The lanes were filled and the runners took their positions. Miller's mind raced: Would John would be OK in front of hundreds of cheering spectators? Would he would trip? Would he even finish?
But as soon as the race started, her fears melted away, and all Miller saw was an athlete, her athlete.
"I just wanted to see the best performance. It seemed much more significant than whether he would ever speak or respond to me," she said.
John ended up winning the gold medal for the 800.
Sports completely changed John, and Miller said that is because of Hancock. "A person with a disability can feel hopeless and isolated, and I think that can happen to parents, too. Jeff probably saw that with us," Miller said, thinking back to when Hancock first met their family.
"John was never expected to be an athlete," Miller said. "But if John could speak, he would say his coach always expected it."