Kieran got his first four teeth around 5 mths. He hasn't broken the skin, but he has bitten me quite a lot.
The first bunch of times it happened, I said, "don't bite!" VERY forcefully, packed up the boob and handed him to dad. I still do that most of the time when he bites, tho it is rare now.
I remember that panicky feeling everytime I latched him on, "will he bite me this time?"
Kieran is now 10mths, too, and he's got 6 teeth total.
I would think that your son is testing those teeth out. The best advice says to do what I did: say "no biting" in your best "no!" voice, take him off the breast and put it away and if possible remove him from the eating context. He doesn't get it right now, but he will understand eventually that to bite means that he can't nurse. Even tho it's incredibly painful, if you can put yourself in the same mindframe as when he was a newborn and trying to establish nursing, that it's going to be tough for a little while but the rewards of hanging in there are great, then you'll get through this phase.
You have done a great job getting to 10 mths when a fraction of nursing women rarely make it to 6mths. It would be great if you could find a way to hang on a few more months and then you wouldn't need to use any formula at all. And, I wouldn't be a card carrying breastfeeding advocate if I didn't also add that perhaps, if you can hold on just a little longer, until the biting thing fizzles out, you might be able to extend nursing a little longer than a year (tho maybe you'll want to day wean or night wean or phase out a number of nursing sessions to suit your and baby's lifestyle better.)
Oh, I almost forgot, how much solids is your son getting? That would be a major factor in weaning (partially or fully).