Biting is serious business. When he nips at you or tries to bite, tell him "No!" in a firm tone of voice and jerk the lead rope. Be ready to block him with your elbow if he tries to bite. Hold your fist up near his mouth so as he swings his head around to bite, he bangs into your knuckles. When your horse acts like he's ready to bite, have him do something incompatible with biting, such as back up or bow on command. Try to squelch bad behaviors before they happen.
For more information, please read the story How to Cure a Horse that Bites