What is the horses behaviour like in a herd with other horses? Do you have access to water, like a lake or something you could take him in??? If so I would def. Go with him in the water, chest high and make him go forward, there is no way that he can rear badly and the bucks will be hardly noticable, been there, done that... water works wonders if you have a nasty horse!!! The dummy idea is another good idea - been there but did take way longer than the water method which is actually the way the natives used to break their horses!