He probably isn't strong enough to carry you at the canter. Believe me, ALL gaited horses will both trot and canter at liberty. My two KMH's do it all of the time in my back yard, but prefer to amble when on the trail. Why do you need him to canter?
I had a farrier who called TWH's "rubber legged." Often you will see them cross one back leg over the hock of the other leg. You don't see other breeds do this. They often get their legs tangled up and can have a fear of cantering when carrying weight. A running walk is really a very fast shuffling walk, and easy for them to perform, having done it their first day of life. Still, all of the TWH's and TWH crosses I've owned would willingly canter when their herd members were passing them. Gaited horses are never really the fastest out of the block.
Build his musculature up, then try again.