If you are lacking in hills, how about riding with someone else. Have them ride a horse that he is friends with or interested in, and have them canter in front of you. If he still doesn't want to, have them pass you at the canter while you are trotting and he will most likely get inspired to play the game and want to canter too. I am all for using whatever method it takes to get him to learn to canter rather than having to run or beat him into it.

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