What was his background? SOme cart horses (amish horses/driving horses) are taught to NOT canter period, since they can't canter witha cart. THey are typically trained to just trot faster and faster to get somewhere quicer, as opposed to upping their gait. I worked witha horse like this for a long time in the roundpen and under saddle in arena. In the round pen, I would kiss once to ask for a walk, kiss twice to ask for a trot, and CLUCK as many times as needed to signal a good canter. It took about tqo weeks to get the horse to even accidentally fall into a canter. Something that actually helped her once she started cantering was to actualy go CLUCK CLUCK CLUCK with every hood beat, kind of keeping her rythym. She caught on on the ground easily.
When we worked under saddle, I did the same triple clucking souns with her hoodbeats, and when she finally went from the fast trot to the super fast lope I just held on and checked her reins to try to get her collected, but I was still clucking to tell her that I still wanted a canter, just a slower one. Now she understands what I am asking of her, and its been about a year.
Anyways, this was just how it worked with this one horse, don't quote me on this "rthym" method, bu the horse caught on wonderfully ^_^
Also, if he is showing discomfort in the bit, maybe see if his teeth need to be floated? If not, then maybe try him in various types of bits. It took me a good 5 different bits with my current horse til we found one that he enjoys. ^_^