Kael Morrison is a rambunctious nine-year-old boy. But until very recently, that wasn't the case.

Many people might recognize Kael from a gut-wrenching ad for the SickKids Foundation.

In the ad, the boy, who was seven at the time, becomes flustered during a shadow boxing exercise with his physiotherapist Ryan Ireland. He pauses, wipes aside tears and perseveres to complete the exercise by punching Ireland's hands, his fists clenched around tiny dumbbells.

Kael was diagnosed at age five with Stat-1 Mutation, a rare immune-deficiency disorder. It effectively decimated his immune system. It was possible he would not live past 12 years old.

There are approximately 15 to 20 people with STAT-1 deficiency in Canada, mostly children, according to SickKids Hospital. Some 100 children are affected worldwide.

This is the first photo of the Morrison family after Kael was released from isolation. (Courtesy Morrison family)

Doctors said Kael's best chance of survival was to undergo a bone marrow transplant.

Luckily, his four-year-old brother Eamon was a match. Eamon didn't hesitate to give his big brother some of his "soldiers," as their mother Emily Morrison said.

"There was no question in his mind," she said of her younger son.

Kael and his younger brother Lochlan share a moment. (Courtesy Morrison family)

A year spent in hospital

Kael underwent the bone marrow transplant in Nov. 2013. The road to recovery was a long one with complications from the operation keeping him in hospital for a full year.

He spent six months in isolation due to the threat of infection and wasn't allowed skin-to-skin contact with anyone, including his mother. Visitors and nurses had to wear protective gear just to sit in the same room with him.

For the little boy, every day was "boring." For his mother, this period was particularly difficult, she said.

The Morrison family gather in a February 2015 photo. From left to right: Eamon, Andrew (Dad), Charlotte, Emily (Mom), Lochlan, Kael (bottom). "When we were able to lift that restriction, you can't even describe it, being able to touch him skin-to-skin and give him a kiss and tuck him in bed. It was pretty awesome."

Returning to the hospital gave Morrison the chance to put Kael's progress into perspective.

"There are days we had to talk to him about just getting out of bed," she recalled. "Now it's 'slow down, don't run around the house with your boots on.'"

Emily Morrison's son, Kael, is the first child in the world with Stat 1 Mutation to be successfully treated with a bone marrow transplant. 0:54

'A remarkable recovery'

Kael returned to SickKids for what was believed to be a routine checkup Wednesday.

CBC News tagged along with the nine-year-old and his family and our cameras were there when he received life-changing news.

Doctors at SickKids say Kael's immune cells have been completely replaced by healthy immune cells, making him the first child in the world to receive a successful transplant for this kind of disease.

A bone marrow transplant allowed doctors to completely replace Kael's immune system. 0:35

"This means he's out of the woods and he will be able to return to completely normal life," said Dr. Eyal Grunebaum. "This is a remarkable recovery."

"I think it stands to reflect the amazing work that was done here by all members of the team," he said.

The news felt surreal to Kael's mother.

"I'm not letting it sink in right now," she said. "You kind of go numb and you live day-by-day and minute-by-minute to get through it."

"To see him here now being able to play, like a normal nine-year-old, that's really all you want as a parent," she said.

Kael plays with therapeutic clown A. Leboo during a recent visit to the hospital. (Courtesy Morrison family)

For more on Kael's recovery, tune in to Our Toronto Saturday at noon and Sunday and Monday at 11 a.m. on CBC television.