"I was trying to find different things for my kids to do, besides sitting at home and watching TV, and then I saw the ad for the Sanitarium TRYathlon on Facebook," Lucas' mother Phyllis Wallace said.

"Thinking it would be something my eldest, Blake, would be into, I asked him and he said: 'Nah, not really.'"

Then right there at their dinner table in Queanbeyan, she entered persuasive mum mode. Phyllis floated each leg of the TRYathlon: running, swimming and cycling. While Blake didn't budge, Lucas began excitedly jumping up and down in his walker, pointing to himself.

For a little boy who can't speak, this was huge.

A few weeks later, Lucas and his mum have become the ultimate mum-son duo. He's got a new special needs bike, is swimming laps at his weekly lessons and is tossing up between using his wheelchair or walker for the TRYathlon run.

It's both his love of the outdoors and appreciation of sports - you can find him cheering on the Raiders or at his big brother's footy matches - that's keeping him in the game. He's also got a cheeky kind of determination.

Lucas has spastic quadriparesis with athetoid movement, a condition that affects all his limbs. His speech is also impacted, so the Wallaces use an adapted form of sign language. He also uses a touchpad, which the family regularly add words to each week.

"He's cheeky. Very, very cheeky. He gets in trouble in class for always trying to chat with his classmates during lessons," Phyllis said.

"He'll come up with little jokes on his communication device, and it just stumps you. You'll look at what he has written and think: 'What?!' And he will just crack up laughing. He's always laughing."

The Sanitarium Weet-Bix Kids TRYathlon is coming to Canberra on February 10, 2019. Last year, more than 14,000 kids across the country participated.

Kids aged seven to 15 can join in, regardless of ability or experience.

In Lucas' age group, there will be a 75-metre swim, 3km cycle and 500m run. For the older kids, they can expect a 150m swim, 6km cycle and 1km run.

Joining Lucas in his age group will be nine-year-old brother Blake, who had a change of heart since his "nah" response at the dinner table. Mum thinks his little brother had something to do with it.

Sanitarium Weet-Bix Kids TRYathlon. February 10, 2019. Early bird discount tickets are available until December 16, 2018. Visit tryathlon.com.au.