Sore on his tongue

I know they're not very good pictures but it's hard to get him to stick out his tongue most of the time. A few months ago he had a little flat painless red circle with a white outline. It didn't hurt so I forgot about it. It still doesn't hurt but I'm starting to get a little worried since he still has it and it's a lot bigger than what it was. Anybody have a clue as to what it is? He'll see the doctor next week but I was just curious.