So the surgery and vet visit went great yesterday. For the most part, the tumor looked much much worse than it really was. It didn't invade bone or any of his teeth (if it did, they would've had to have removed part of his jaw) and was able to removed almost completely. The doctor suspects it may've been something like an epulis (a usually benign tumor of the mouth), but we won't know until the biopsy comes back in a few days....