I REALLY like what MIEventer said. Wonderful post!
I'd take some time and work without jumps on trot/canter/trot/walk transitions, with very few strides of canter in with lots of walk and trot, working up to more and more strides of canter when he masters that downward transition.
Once he's mastered the calm, cadenced canter, go into an arena with maybe two small crossrails max. Ask him to trot to the jumps, land in canter, and come down to a trot a few strides after the jump. Mix it up... canter to the jump, land it, come right down to trot, etc etc. Make him think, don't let him get comfortable and anticipate the same routine. Reward good behavior, and don't yank on the mouth for bad behavior, ignore it and try again.
It might be a little boring compared to what you want him to be doing, but remember, he's got a problem and it MUST be fixed for him to be a safe ride.
When he can be sane to and after a simple x, try a small vertical. Incorporate lots of grids and gymnastics, REALLY get that mind working.
You can do it!
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