For motivation, he tacked a piece of paper on his bedroom wall which said "I will make it into the New Zealand Surfing team". Today when Stewart reflects on having made that goal a reality earlier this year in Japan and finishing second in the 2018 New Zealand Surf Series, he does not rank these as his most significant achievements.
"I am just grateful to have my health back and be able to walk and to reach surfing at a high level again.
"I guess I just wanted to prove to myself that I could do it."
Professional surfing is his life, he says.
"I love it."
He is at home and most comfortable out on the water.
"I have the freedom to do whatever I want. On a bad day you go out there, and it all goes away."
But that is not what drives him.
For four years he has travelled to the United States and Australia to work as a volunteer for Heal Surf which teaches autistic children how to surf.
"Competition is my life but trying to give back to the sport is more important to me."
His role models are his former Raglan Surfing Academy coach Larry Fisher who was "crazy good to me" and the "closest thing to a father figure".
But the 23-year-old is a mother's boy at heart.
"My mum is the biggest inspiration in my life and getting to hang out with her is pretty special."