My old trainer used to carry around a safety pin and every time a pony would offer to bite let him/her run into the pin. Or when it tries to give it a little prick on the side of the muzzle. This will make the horse think that when it bites you it will hurt him/her.
Try not to hand feed him anything. Feed him any treats in a bucket so that he doesn't associate your hands with eating
Try to catch him before he does it. If you can't catch him right before he does it make him back up, turn away, or do some sort of work. I would also do the same if he does bite so that he knows when he bites he ends up having to do a lot of work.