There are three things that seem to contribute most to biting:
1) teething - so try and give him something like a cold facewasher to suck on before you feed
2) imperfect attachment. Although it seems very counterintuitive, the more boob you have in bub's mouth the less they can bite, and the less the bite can hurt. Also, support bub's head really well, so they don't feel they are slipping and clamp down.

If neither of these work then it is likely to be
3) curious babies working out what their mouths are and aren't for. Say something like "No biting Mum", stop the feed, and pop bub on the floor to be sad for a minute. It usually only takes a little while for them to get the message

If you feed lying down and support him behind his back then although he may bite he can't bite and seriously pull back as well, which hurts the most

good luck - the ABA has some really useful info on biting too.