Maybe try a half seat when u ask.
Make sure you don't rock your hips and get into a light seat.
His saddle might not fit properly
Hi, If he bucks every time you ask him to canter it may be a good idea to get him checked over by an osteopath. He could have a bad back.
He could be stiff. Or you might be kicking to hard or pulling on the reins when you ask
My horse used to buck when I'd hold his mouth too much when I asked him to transition, so maybe try letting him stretch a little when you ask him to go up to a canter
He could be stiff, or just feeling excited, so just try to work through it and not make a big deal out of it because then he will too:)
Ignore it and just keep going. He will get bored of it 😂👍🏼
Maybe needs his back cheaked !!