I can't stop fantasizing about my friend's son.
How old is their son and what kind of fantasies are you having?
I'm thinking about fucking him. He's of legal age if that's what you were getting at.
It's normal healthy to have sexual fantasies about people, but some things should never be acted upon. Try not to blur the line between fantasy and reality and you should be okay.
Well too late for that. I already fucked him.
Well you said they are of legal age, so there's obviously no legal trouble. However, if your friend find out what you did, they would almost certainly be very upset with you. It would probably be for the best if you talked to their son and told them you can't continue this relationship anymore.