Small spurs can also work wonders. I rode a lazy-ass lesson pony in 1/2" spurs and he was wonderful. When I wore them. The second I tried to ride without them, he'd fall back into the 'slug' routine.

Also making sure he actually understands 'canter' is important. I've got a part Appy, and there's a fine line between "i don't get it" and "i don't wanna". Make sure you pay attention to him so you can get to know when he's truly confused and when he's just being a dick. It took me awhile to figure it out, but it made training much easier when I did.

Oh, and don't ever be afraid to get on his ass about something. I've found Appys to be pretty smart able to push their owner's buttons easily. Once you're boss though, life is easier.