Breastfed baby "biting" Help!!

My son is 4 months and he's started teething the last week or so. So much drool and chewing on everything! This past week he's started "biting" me while breastfeeding (he has no teeth yet, but it's very painful nonetheless).  I've tried pinching his nose when he does it, but it doesn't seem to be helping. He just gets upset and then bites again when he latches on. Any ideas on what I can do to stop/prevent this?