After gruelling rounds of surgery and chemotherapy his devastated parents, Steve and Hilary, were told last year that he was dying.
Not willing to accept their son’s fate they began researching alternative therapies.
They came across a non-psychoactive cannabis compound called cannabidiol (CBD) which some have claimed has cancer curing properties.
Steve and Hilary found a private clinic willing to prescribe the synthetic substance.
Within a few months they say William’s golf ball-sized tumour had shrunk by two thirds.
Now William, from Newark, in Nottinghamshire, is finally well enough to go to school part-time.
Steve, 36, and Hilary, 34, are now backing new research by experts at Nottingham’s Children’s Brain Tumour Research Centre into whether CBD can reduce paediatric brain cancer cells.
Steve, who is also dad to one-year-old Charlotte, said: “In late 2013, we started noticing William’s head tilting to the right-hand side – no one had ever seen anything like it before.
“His balance was off and he started vomiting randomly so we took him to the GP.
“After a few days, they realised something was quite seriously wrong, and did an MRI scan – that’s when they discovered a tumour the size of a golf ball.
“It was a highly aggressive, grade three tumour – and the prognosis from the doctors was very poor.”
William had surgery to remove the tumour but 18 months later, after nine months of chemotherapy, the tumour grew back.
He was in intensive care for six weeks before having radiotherapy – but still it didn’t have an effect on the tumour.
Steve, a civil engineer, added: “We couldn’t bear to accept there were no more options for William.
“We were desperate and willing to try anything – when we read the research on cannabis and tumours it gave us hope.
“It cost us £2,000 for a nine-month supply, but we wanted to make sure we were giving William the best quality stuff – we gave it to him diluted in water.”