Bigger circle, letting him go faster, not going into the corners of the arena, getting out of the smaller circle such as a round pen-type setup, more urging.
If he does not pick up the canter on the longe line at the word 'canter' there's not much point in trying to get him to do it under saddle. You say 'canter' when you're longeing him, then of course if he doesn't canter right when you say canter, you tap him with the longe whip on his hind legs above the hock or just on his rump more behind him, to urge him to canter. The first couple times on the longe they tend to trot fairly fast and finally canter, that's fine.
So don't try to canter under saddle til he knows the word and does it every time you ask on the longe line in both directions.
You just take what you taught him on the longe and basically do exactly the same when you are sitting in the saddle. Say canter, tap with your whip, he canters.
Oh and if you try to take your young horse off the circle they tend to fall back into trot, you do better if you canter on a circle just at first.
Hope it works out for you.