I would have to say that, its hard to tell without seeing this horse in action. Does he feel unsound to you? Does he protest in any way? Give a little crow hop..kick out...buck...stall completley? I ask this because all of these reactions might better point to an uncomfort problem. If this is more of an issue of "working really hard to get him to canter" it might just be a lazy and lack of balance thing. Often a horse who is unsure, unbalanced will be either lazy for the sake of not working OR just simply give you a hard time and is simply being stubborn. Its hard to say. Perhaps have another set of experienced eyes watch you and him go and see what their input is. If he in unsound or it leads more in the discomfort direction, Id maybe give your vet a call and talk to them about it. It sounds to me like you might just a bit of a stubborn/lazy problem though!
Happy Riding