Well, I don't see why he wouldn't think you're dating that guy. Most shy guys would hope that they never have to see the girl they like with another guy. But he saw you with another guy! And you had him by the arm too! I think this shy guy of yours might want to back off now, so you should take the initiative. You could just ask him out some time, and if he asks about that 15 year-old, tell him what happened! Good luck!