Could be a type of fungus. I would see a dentist if it gets worse or if he has other symptoms associated with it.
Oral thrush is typically white not brown in appearance.
Hi, I use to work in a hospital and I don't think it is any thing to bad. It sounds like it could be trush. It is a type of fugues. And it can be taken care of by medicine.. Just make sure he goes to the doctor before it spreads to all over his month... Let me know how it goes...
He should go and have them assess by a PHYSICIAN.