Maybe I missed it, but when did you start him under saddle? How long have you been asking for canter work? Is he good for your trainer and on the lunge?
I think I did not include that. We started ground work in may and at the end of August we started under saddle. We worked simultaneously on ground/voice/lunge commands while I was doing w/t in the saddle and started canter work from the ground then as well, with an emphasis on downward transitions and voice commands.
We took him on a few very simple, easy trail rides in October after we moved barns (and he's beautiful on trails, listens like a dream, not spooky at all, a complete angel).
I was riding 3 days a week in October and I think mid-October we did our first canter, one rotation of the arena and then started to work that in gently as he's still getting his topline, gaining weight, etc.
He's great at everything as long as third gait isn't introduced or allowed. We do pole work on the flat, figure-8s, serpentines, etc, and he enjoys and is very focused when given something that he really has to think about.
It just seems like he STOPS THINKING when third gait is allowed, and once that happens, it takes almost an hour to get him to settle back down so he'll listen.