First it was the cerebellum that shrank -- the area at the top of the neck controlling movement and balance. But over the past few years the nine-year-old Australian boy's entire brain has compressed, and nobody knows why.
There are no other recorded cases of this mysterious brain degeneration in the world, leaving Royal Children's Hospital (RCH) specialists in Melbourne to continually rewrite the textbooks on his treatment.
And while the undiagnosed condition means Jason has never been able to walk or talk, what he has mastered with remarkable strength is showing emotion and giving unconditional love.
His connection to the world is evident when his brilliant blue eyes light up when someone new enters the room.
"There is no cure, no tablets or medicine they can give him. All they can do is give him the most comfortable life," said dad Jamie. "And at this stage he's 100 percent a happy boy and loving life."
Jason showed no signs of degeneration at birth, but when he was not crawling or walking at 12 months, Jamie and wife Terry knew something was wrong.
At first they tested for common and treatable conditions, but as each result returned a negative doctors were forced to spread their probe further.
An MRI brain scan showed that the cerebellum had been eaten away since birth, and by age six he was losing skills such as standing and his ability to feel pain.
But the next brain scan was the bombshell -- not only was the cerebellum disappearing, his entire brain tissue was reducing.