If it bothers you tell him you know and ask him to stop. If it isn't a problem and you are just watching to see what he is doing then don't say anything. I got news for you though, he is probably going to mad that you are watching him, he isn't going to stop he will just find a way to do it without you knowing. I know it is hard but you might have to find a way to deal with it. I would set clear limits on the chat rooms though.