Also, when the biter was a baby I did not own him, a neighbor did. I would not allow him to bite me. She thought it was cute and allowed him to bite her. I have been dealing with this ever since then. Any help would be appreciated.
Horses bite to play or assert their dominance.
Here's what I can offer. You can use one of several methods. If you do it mild to medium, he will interpret it as in invitation to play or it could make him head shy. So mild slapping makes things worse. Giving the horse a knuckle sandwich isn't usually the best answer either although with some horses it would end the problem with one application.