Well, no, I wouldn't just accept it.
Early teenagers don't have a clue about what their bodies are doing, and how they're making them feel, for one thing. They go through all sorts of fears and emotions and the like in the course of maturing.
I think peer talk and the overwhelming emphasis on homosexuality causes teens to grip onto it, like all teens do with any fad. (We all did, remember?)
I would address it as a side issue, not with any upset or excitement. I'd make sure to give him a book, even if from the library, on the human body and how boys grow and mature and the likes of what goes on with the hormones and the brain and such.
I'd tell him he has plenty of time to determine what he wishes to be as an adult, in the future.
I would accept him, and his fears and anxieties, but I wouldn't accept the idea as fact that already he's homosexual.