What does he do when you ask for a canter? What is his response?
Does he canter when you lounge him? If you don't lounge him i'd work on voice commands while you are safely on the ground. That established the language your horse and you communicate with. Make sure that he understands what you are asking for by voice. That's the way I break new horses and I rarely if ever get thrown and if I do get thrown I know that I didn't do the ground work like I should have I rushed the horse.... but that's me. I'm an OLDEr GAL AND I HATE BEING BUCKED OFF HORSES :) my guys are all pretty good size too LOL