Transitions are your new best friend :) do a ton and a half of transitions - upwards, downwards, change it up. This will help him balance throughout the gaits. One of my favorite exercises is doing 10 strides of one gait, 10 strides of another, and back. This can be 10 strides walk, 10 trot, 10 walk, 10 canter, 10 walk, 10 trot - get him responsive to your aids.
Don't think so much "slow" as "collected" - a "fast" horse has energy rushing out the front. Being that energy back to center by closing the front door and containing the energy; getting the horse to work off it's hindquarters. There are a ton of threads on collection - look for posts by Anebel, Maura, and Kayty in particular. There are other members I want to mention too but their names are escaping me right now.. Agh!
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