Good questions above.
Is this consistent or just something the horse has done once or twice? How are the bugs right now, do you have horse flies?
Not the only way to do it, but my thought is if the horse is trying to get out of something by bucking I'd work his butt off every time he does it. I'd want him to realize this is pretty easy, and if he is going to act up it is going to be so much more work. I'm not talking about being abusive, but using a horse's natural tendency to be lazy to my advantage for it to be his idea to behave.
That's worth what it cost you though because we'd need some details to get an idea why your boy is bucking. Good luck!