Fix it on line. It's a timing and feel issue. Tell him to go (into the canter) but not like that. The signals are so close that most people think they can't give them, but that's the reason to do it on line so you can watch the horse and know that you were late. Plus, you won't have to ride the bucks out, you can do that on the ground. It will take you some time to get your timing right and then you will have to give your horse time to figure out what you now mean, but you'll be a much better horseman for taking the time to figure it out.