biting!

How can I get my 4 1/2 month old to stop biting during feedings? His two bottom teeth popped through just a few days after he hit 4 months and they're razor sharp. He wasn't really biting very often before but today he's bitten me every single feed and I have no idea what to do. I'll scream in pain when he bites and tell him no and he just laughs and smiles at me obviously, because he doesn't understand he's hurting me. What can I do?