Help biting breastfeeding

My baby is biting my nipple! I yelled the first time in pain baby let go confused second time I put him in the crib and said no third time I am in pain fourth time today looks at me bites down then breastfeeds, my nipple hurts

Baby is 6 months old with two bottom teeth and I just don't know what to do with him biting he even smiles when he does it