Try it on the ground, stick him in a chute and send him over a few jumps and see if he bucks without you on him. If he does then he has a lot of energy, if he doesn't its either pain in his back (possibly due to saddle fitting, or maybe the way your landing on his back) or him just being a tool. We have a horse that does that and its because his rider quite honestly isn't graceful and he is sensitive, not a very good fit. So after each jump he throws a mini buck with her. Maybe he should have someone else ride him and see what he does, if thats not the issue which it probably isn't then you should have his back checked out, saddle fitted, hooves checked, have the vet basically scan him when you ride to see what he/she thinks. We have another horse that used to knock a rail and throw a hissy fit and it was just because she didn't like the feel of the jump touching her. Do you use any sort of boots? Maybe you should use front boots to give a lil cush or polos, not open front boots, they aren't going to solve it if he's knocking the rail with his front feet.