The other posters here sum up my immediate thoughts perfectly. 15 is a bizarre age, to say the least. If he's a homestay, does that mean he's been separated from his parents? That could explain a lot, if he came out of a bad home environment or something.

If you want to be nasty (which might be the only way to go to), tell him you're going to get your boyfriend to beat him up. If your boyfriend isn't a big guy, tell him your boyfriend and a bunch of his friends are going to beat him up. Sometimes you have to fight fire with fire when it come to teenagers. Of course only do this if you feel secure enough, but it's just a thought.