I must have missed this post when if first came up. When does he tend to bite? Is it when he is drifting off? Or is it more playful? Or due to frustration and pain on his part?
Most often, babies bite as they fall asleep. In that case, you could try to pull him closer to cover his nose, so that he lets go right away.
If it's for other reasons like that he's playing or using you as a teether, I agree with Diane, you have to set a boundary. This might be baby's first lesson in discipline. Babyjama was doing this for a little while. She would bite down and then laugh when I said no. So I would put her down and tell her that Mama wasn't nursing her if she was going to bite. You don't have to put them down for very long to get the message across. She got the message after a couple of days.