J.S.
Hi Juliet,
Is it possible that he is nursing with a different latch because of his tooth? Maybe the normal latch makes his gums hurt?
I'd try the following:
Everytime you feel his tooth, unlatch him (with your finger) and then say "no teeth", and start over with a new latch. Make sure his mouth is open wide and he has his tounge out a bit before you let him latch on. You could also try a different hold.. lay down with him, or try the cross cradle or football hold.
Good luck, and let us know how it goes!
J., SAHM to 2 year old Charlie and 5 month old Joey