I'm also wondering if it is pain related. Have you palpated his back? Especially check the top of his rump/over his hips and in the SI area. Even if he has no reaction to palpation does not mean he is not hurting.
A very high percentage of OTTB have lameness issues.
This site has some excellent videos on unsoundness issues in OTTB. Maybe see if any of these horses look or move similar to yours? Horse Problems Australia talks about stifle problems in horses and their reprocussions
I have a horse who bucks/crow hops etc. It is a soundness issue for her. Anyone else would have beat her for it and assumed it was an "attitude problem". Her lameness is a grade one at the trot (after the vet does flexions). Without flexions you can't tell she is even lame at the trot. Her canter however is an absolute mess with crow-hopping and bucking. The vets have not determined what the problem is- her X rays are clean. Their best guess is that this is a problem in her spine or SI probably due to a traumatic injury as a 2 yr old.
I know another horse with a "bucking problem". In his case it is also his SI/back. He cannot be ridden and is used as a driving horse.