Spyder, let me re-phrase the question asked. How did you TEACH him to canter when just started training him? I'd assume the aids should be quite exaggerated in the beginning.
The rider also needs to see if the horse is more "straight" naturally as some are and not overdo aids, esp the outside leg so they don't unintentionally cause the horse to become crooked.
Ideally the outside leg should "guard" against a swing out...not become an active agent unless needed.