H.P.
My inclination is that his latch has changed due to the tooth coming in. I'd talk to a lactation consultant or La Leche League Leader to see if they can help you out. My friend found out that WIC offers free breastfeeding consultations even if you're not receiving benefits, so that could be another way to get advice.
I found that when my daughter had one top front tooth it left quite a divot in my breast - since it poked out further than her gums it tended to leave a good mark on my soft breast tissue and it was at times painful. As soon as she got additional upper teeth in it resolved, and now (at 14 months) she's got 12 teeth and it's rarely an issue. Every once in a while she's lazy about how she latches on and will bite down a little hard, but I just detach her and either leave her alone in her crib or a safe room for about a minute if I think it was her fault she bit, or just reattach her if it was my fault. So get some help, stick with it, and hang in there!
HP