This sort of problem tends to come from pain &/or fear. So first & foremost, rule out pain/discomfort. Saddle fit is the biggest general issue, as it's very common for saddles to be restrictive, too narrow, too long, etc & when the horse tries to collect himself for a canter, this body shape change can make the saddle more uncomfortable. I'd also want to rule out other back issues too. Fear may have come from his initial training being too rushed or such, that they never got him confident cantering under saddle so even if he is so now, the associated 'habit' has remained. If you suspect there may still be some fear in it now, I'd go back & ensure he's comfortable cantering under saddle without a rider first, before building very gradually up to the point of cantering under a rider, ensuring he's truly confident with everything along the way before progressing further.
You don't say how long you've had him, but trouble is, the longer he's been doing it, the harder/longer it will be to change the behaviour, as the more established it's become, as part of a pattern. I would suggest something such as ruffian's advice - do what it takes to make that behaviour difficult & uncomfortable for him before trying again. Over & over. It could take a fair bit of repetition for him to finally start a canter without crowhopping - might take until he becomes tired - but I would budget for the first few sessions being able to spend as long as it takes to get it to happen before you quit. Ensure that when he does do the Right thing, you reinforce it instantly, negatively, by removing pressure, quit asking him to work, and preferrably(IMO) positively as well, with a reward.
After he's cottoned on to the idea that going straight into a canter is what works for him, then it should be much quicker for you to work through the hops & get what you want from him. But you will have to be utterly consistent & persistent. It will likely take quite a lot of repetition over a lot of sessions for him to start to become solid - depending on how long this behaviour has been allowed to go on.