I've ridden a lot of horses that get extremely excited into the canter. Its basically going to be difficult because its all part of his training. When you get ready to ask him to canter, relax a great deal, fake it if you must, deeeeep breaths, because the calmer you are on him, the calmer it should help him feel. Then just before you ask him to canter fall this......half halt, canter aids, half halt. This way before you even canter you are asking him for your attention. Then youll begin to canter but you're again asking him for your attention. Work on transitions witht this. Canter for a few circles and then break back to a controlled trot and repeat. Eventually he should understand that theres not a need for so much extra excitement in the canter and he should pat attention. This isnt a quick fix at all, but youll have to rid it out, once he starts getting excited, move him in smalllll circles and calm him and break him to that trot and try again. You really want to practice the transitions for him.
I know this helps out in the long run for you. IDK how quickly itll help though, but I wish you so much luck with him