Hm. Well, rule out physical issues, first. After, check your tack. If both of those aren't the issue, then I would investigate his feed- what does he eat right now? Theres also your riding- is it you? I see that you've eliminated physical discomfort, but I would investigate his feed a little more- ask for the ingredients, compare to the other feed offered, his old feed, and if it comes down to it, I would ask about getting your own feed. Do they really make you use only those feeds? Just curious, I've never boarded a horse. :)
These are all factors to investigate first, and then if it's just him literally refusing to canter while being ridden because he's just plain lazy, you need to make him snap out of that. He needs to know that you mean business. I seem to find that the majority of people's problems when it's not tack, a physical issue, or their riding, boil down to a horse that for some reason has no respect for a person.