When I am dealing with horses that bite (or very young horses that have not yet learned manners) I want them to feel like the discipline is directly related to the biting, that they directly caused it to happen. Try paying really close attention next time you are with your horse. When you see his head start to come towards you, flick out your elbow and pop him in the jaw. You want him to think that he is running into your elbow. If he does get a hold on your skin and actually bites you, INSTANTLY make him work HARD. Use the end of your lead rope to move his hindquarters, forequarters, back up, whatever you need to do to make him feel really uncomfortable. As has been stated, make it short and hard and unemotional. Then, go back to what you were doing. You want your horse to finish moving and think, "What do I need to do to make sure that doesn't happen again?" After a few times, he will make the connection between the bite and the work.