Horses learn through consistant repetition. The same thing everytime=the same responce. So with time you can teach any horse anything. It is how you should work with your other horses as well. We always use headers with our drafts, but its not neccessary, just easier. He may not have had one, but may have been allowed to walk off when mounted, or been kicked when trying to get on, or just not made to stand, reasons vary. You decide what behaviors are important to you, and make them become important to him. If he is not use to a bit, I would have his teeth looked @ before putting one in there.