I have an 18hh percheron youngster (4 yo) with a hell of a buck that comes up just about every time he's going under saddle.

He's in training and we are discussing all the possibilities for getting him to stop doing this.

What are your tried and true ways of solving this sort of issue?

Assume he's past all of the world's most expensive vet tests, has a nice saddle that fits him well, is growing and his tack accounts for his growth, has all of the best snacks appropriate to his ever more giant size, lives turned out, has had his feet done, teeth inspected, insides inspected, outsides inspected and has no other structural issues. He was going in a snaffle but that proved insufficient - he is now going in a 5.5" Mylar with a 1" shank.

At age 4 he cannot trot a full circle around the arena without causing havoc.

He's a nice boy on the ground and very food motivated. Otherwise aesthetically lovely. I am concerned for his future if we can't fix this - he's already bucked me off breaking bones and I'm grounded for four more months. I have ridden horses for 23 years.

Out of the box thinking welcomed.