Bucking Thoroughbred?

It depends on what type of buck it is. Like if he's just doing to to get you off of him, or it could show he's playful and feeling good. If he goes to put his head in between his legs squeeze him don't pull on the reins because when they go to stretch your don't wont to discourage them when they put there head down squeeze to keep him moving forward. I would also have a trainer look at your bit and make sure your using the right one. Does he have something wrong with his back? If is not any of the above I said you should look into it.