Would you like this guy to hang around with you? If yes, slowly bring him into the group and help him communicate with you better. It sounds like this guy's been alone for so long that he just doesn't know how to relate to other people properly, even if he wants to be your friend. If you don't want to hang around with him, I would suggest finding him some different friends, and if none can be found, let him down as easy as possible. It could very well be that he's got some mental thing going on that being too forceful in telling him to GTFO could set off, and that could lead him to harm himself, others, or even you. The majority of school shooters have been lonely, antisocial people who just snapped one day. Friendly companionship is one of the best preventative measures for that I can think of.