Tell him to bring a separate tent and pitch it far away and downwind. Tell him straight out he has a problem that could lead to his having to move and that you expect him to take reasonable efforts to remedy it. That would surely include opening windows, trying to find the dietary or metabolic reasons etc. Also, the fact that he is free to walk around in his underwear doesn't make him sound like too conscious a roommate. I think you need to sit down and set down some expectations with him and if he doesn't like them tell him he will have to move out.