I might have asked this before, and I certainly don't know much, but could it be that he was never trained/allowed to canter?
I have a Fox Trotter mare that I can only get a proper canter out of half the time, normally going up hill because it shifts her weight back over her hind end. On the flat ground she will often swap leads or do a pacey thing rather than a proper canter. I get the impression that is common with gaited horses.
If I've already asked that before (I don't remember) then please disregard this post. It also could be a combination of both....a lameness issue along with a horse that doesn't know how to canter.
Does he ever canter freely not in a round pen? Like a straight line in a field or arena? Does he move the same way?