I have almost exaclty the same problem with my horse Dinado. I did a lot of lunge work with him at the canter, with and without a saddle on. Eventually I had an expierenced rider get on him, still on the lunge line, and worked with him until he would canter with her on him. Eventually I was able to get him to do it even off of the lunge line. He is now a fantastic dressage horse and is moving along great, he still pitches a fit every once in a while but you can't be perfect haha. Just be patient and work him as often as possible.
Good Luck! :)