Hey! I'm a breastfeeding counselor who breastfed for two years, so I know how you feel! Definately let him know that biting is unacceptable. And he does understand when you tell him no.
The best thing to do is watch. Does he bite at the end or beginning of the feeding? If it's at the beginning, try pumping or expressing some milk before you put him on the breast, rub his gums with or without the breastmilk, just to relieve some of the discomfort. But usually it's at the end. Watch him and you will start to notice when he's done actually nursing. That's usually when teething babies bite down. Start taking him off the breast before he gets bored and bites. You'll be able to tell when he's actually done.
When he does bite, immedietly take him off, don't yell and startle him, just take him off, tell him, "No, don't bite mommy." and hand him a teething ring. Lay him down, and walk away, so he really knows what's going on. A few minutes later, you can pick him up and try to nurse him again if he didn't finish eating, or just pick him up, and love on him so he knows you're still there and aren't mad.
After a couple of times, he will start to take the hint that biting means not only no food, but no mommy for a few minutes!
Remember, he isn't biting to be mean. He's biting because he's hurting, uncomfortable and having to learn how to nurse with these new things in his mouth. He'll get used to it, just keep it up! Don't give up. He will stop biting, I promise!!