But his mother, Linda Roberts, 47, saw someone very different. "I saw a little boy who shouldn't be here," says Linda. "He had come so far from the day in hospital he nearly died from meningitis."
Jonnie, 19, was the much-loved fourth child of Linda and her ex-husband Chris, born on 28 May 1993.
"As a little boy he was full of energy, but he was always teasing his sisters," says Linda.
With three big sisters, Bethany, now 28, Rebekah, 22, and Hanna, 21, Jonnie was either going to be a big softie or learn how to stand up for himself.
"When I did the girls' nails and make-up for the school disco, Jonnie would insist on getting his done too," laughs Linda.
When Jonnie was five, Linda and Chris split up, but the family was to face an even bigger trial.
On Saturday 11th October, Jonnie was suffering from a temperature and was sleeping fitfully being sick throughout the night.
"Two of his sisters had suffered with a virus a couple of weeks before so I assumed it was Jonnie's turn," says Linda.
"But by Sunday morning I knew this was more than just a bug. Jonnie was smothered in a bright purple rash that covered his whole body and face. I'd never seen anything like it."
Linda raced Jonnie to Addenbrooke's Hospital, Cambridge where doctors quickly diagnosed meningitis.
By now Jonnie was fading in and out of consciousness. He had been moved to intensive care and had a whole team around him.
"The nurse told me: 'He can't fight this. We have to put him into a coma. You need to say whatever words you want to now'. Just as I was coming to terms with what she was saying, they put another line into him. Jonnie let out a little 'ow'.
"'You stop them doing this to you,' I told him urgently. 'You fight this. I love you.'
"I sobbed, thinking these would be the last words I would say to him.
"I sat next to him and stroked his arm looking at his tiny five-year-old body. He looked so fragile and vulnerable. He was swamped in wires. If it hadn't been for his beautiful blonde hair I wouldn't have known it was him."
Jonnie was unconscious for a week and the doctors warned Linda Jonnie could have severe brain damage.
Finally Jonnie started to come round.
Linda recalls: "'Hello darling,' I whispered. 'It's mummy. I love you.'
"His tiny weak voice whispered, 'Love you'."
It became obvious that while Jonnie was fine mentally, his body was still fighting the effects of the septicemia, the blood poisoning that had caused the angry rash over his body.
"The surgeon Per Hall told me they'd have to amputate Jonnie's leg from below the knee," Linda remembers. "I was devastated, but I knew this was the price I would have to pay for having my boy alive."
It took another month before Jonnie was strong enough for the operation.
I told him his foot had died and we needed to get him a new leg so we'd have take his old leg away. I knew he didn't really understand.
A month later, in mid November, Jonnie was finally strong enough for surgery. It took three hours for surgeons to remove his leg.
When he opened his eyes, he screamed in pain. Then he looked down and saw a big gap where his leg used to be.
"He sobbed that I had given him the wrong medicine," says Linda. "I knew he was talking about the Calpol I used to give him whenever he was ill. Jonnie refused to look at me for the rest of the day, blaming me for the loss of his leg. But as the pain relief kicked in, he became his mischievous self again."
The girls had different reactions to his stump – or his sausage leg – as he liked to call it. While Hanna didn't want to look at it, Bekah would sit and stroke it as she watch TV. Linda didn't stop either reaction as she knew Jonnie would face both extremes through his life.
Six months after his operation Jonnie was fitted with his first prosthetic leg and had to learn how to walk again.
"As soon as he could walk, he started running around immediately. There was no stopping him," says Linda.
For his school sports day, eight months after his amputation, I asked the head teacher if there could be a hopping race, so he could be equal for one thing. But as soon as it began I felt bad. Jonnie stormed it and left everyone behind!
Jonnie had to relearn to ride his bike again, but this time he used one leg, pushing down on the pedal and waiting for it come back up before pushing down again. "He made it seem like a doddle," says Linda.
Of course there were bad days when he couldn't fit his leg comfortably and he would nearly miss the bus. Linda would give him a piggyback ride, running through the village to the bus stop so he could make it on time.
"It made me think of the future and the difficulties Jonnie would face as an adult," says Linda. "I got in touch with the Papworth Trust, a charity which helps families affected by disabilities. I started working for them when Jonnie was nine. It helped I could understand what other families were going through because I had a personal experience of disability."
In 2008, when Jonnie was 15, he applied to one of the Paralympic talent spotting days. Linda took him to the East London event along with Jonnie's step-dad, Steve.
"I didn't give much thought to Jonnie winning any races, I just hoped he could meet other amputees his own age," says Linda.
But Jonnie's ability shone through and he was eventually picked for the GB Paralympic team and began to train regularly.
He went on to win a gold medal in the 2012 Paralympic 100m race, the T44 class aged just 19.
"As he stood on the podium on 6th September 2012 clutching his gold medal, the world may have seen a champion, but I saw a little boy who shouldn't be here," says Linda. "He had been so close to death and now he was a world champion.
He's had such an amazing journey and it proves good always comes out of bad, and that with support anything is possible.