I had the same problem, I went to a clinic and what I learned was to let the horse run but tire him out by doing circles all over the arena. Ever since then I've had no problem. But it takes a lot of confidence and guts. When circling keep the horses inside head to your leg but let the outside have full rein.
Or possibly do you think he is just having balancing/strength issues? Some horses need time working to get strong enough to have a balanced canter they can maintain consistently.Some trot poles, shoulder in, haunches in, half pass, type work can help him develop? its hard to tell why he does that without actually seeing the horse in action.
ride half circles at the canter bringing him back to the trot or walk after every half circle. let him walk on a loose rein after the half circle, then pick up a trot or ask for another half circle at the canter. It helps to keep him focused and prepared to downshift. After he becomes more reliable at the half circle let him go for the full circle. keep mixing it up with half circles or full circles at the canter to keep him thinking. keep the canter work short with lots of loose rein in between. Sometimes let him stop, stand and do nothing. That seems to keep them the most relaxed. It only takes a few weeks of work like this before you should be able to slowly extend the canter figures.