I would rule out pain first. Going any faster than a walk may be pinching him.
Or I would recommend Clinton Anderson's "cruising" method. I had large success with it with my green 5 yr old. The goal is to have the horse stay in one gait, with no assistance from you.
For example when my horse needed to learn how to canter on his own, I brought him into our hay field, got him cantering and let him choose what direction to go, as long as he stayed cantering. He would only be reinforced when he would drop down to a trot or walk. His release would be me leaving him alone, and just let him go. Good luck!
If the weather ever gets nice again I'll give that a go :)
Posted via Mobile Device