has he recently shed and/or fed? BP's briuse and scratch real easy right after a shed or could have gotten scratched during a feed.

Your vet can give you a clearer picture but i just wanted to point out other possibilities. keep a close eye on him for any other possible clues. is there an identical mark on other side of mouth? I have seen marks on mouth that look suspiscious only to see the exact same thing on other side showing it's just part of their natural markings.