Was he off the track? I would work on spirals with him. Work in toward the center and then push him back out again using your inside leg on the circle. Eventually have him go straight for a stride or to and work your way up to more.
Also something I find helps with my horse is not trying to manager her stride so much. If I just kind of let her canter and don't mess with her she seems to be happier and not want to lead changes.