I know it’s hard but you need to try your best not to react. He doesn’t understand that he’s causing you pain, or the difference between a good or negative reaction. Have you tried unlatching him, firmly but quietly telling him something along the lines of “don’t bite mummy”, and then ending the feed for a few minutes? If he’s still hungry latch him back on and offer him a teething toy if he bites you again. He should learn that if he bites you’ll end the feed and he won’t get any more milk. Hopefully this helps 😅
Did anything change lately? Have you introduced a bottle, or a pacifier?