HOw does your horse do in ground work? Does he lead well, does he come up off the rope if you walk or trot off, or does he lag behind? Will he stop or go beyond you when you stop? Can you lunge him around you? Can you have him change directions? HOw good is he at following the rope/feel?
I ask this because it relates to how well he can follor the rein when you ride him.
If he is having trouble keeping his balance at the canter, you might do more trot work , always working on circles. Put some ground poles out and trot him /or lunge him over them. He will be able to canter better once he is well balanced at trot.