Very good point my margypops. This is a learned behavior. Where or how he learned it, is the question. And no he is not gay. And yes, it is experimental, but he should be told that it should not be done.
I doubt at that age its a sign of being gay , you may want to ask him had he seen anyone else doing it,had he seen it anywhere .I think some experimenting is normal but its also good to be aware that other things can happen .good luck .