If you're feeling his tooth in a way that's uncomfortable, he's probably gumming a little bit cause it's a new feeling. And if he bites, it's also because it's hard to resist when you have mouth pain. So in both cases you need to make it clear to him that it hurts you and he's not allowed to do it. When he bites, instead of pulling away, which hurts a lot, push his head toward your breast. It will cause his mouth to open so you can free your nipple. Then immediately put him down and say "No bite!! that hurts mommy!" Then continue and repeat this until he gets it. If he thinks it's funny, and keeps doing it, then give him a break for a little bit and resume later. This should work, maybe not right away, but it will if you stick it out. I did this with my daughter and we were able to continue for a long time. She had a full head of teeth by the time she weaned! Hang in there, I know it's not easy sometimes, but it's worth the frustration and a few little nibbles.