I'm also a very jealous person, so I can understand where you're coming from. I think you should just be honest with him--tell him that it makes you uncomfortable. I have a lot of guy friends, and there have been plenty of times where I've crashed at one of their houses and nothing's happened--we're just friends. But it sounds as if there's at least some sort of attraction between these 2, and I think that would make me feel very uncomfortable. I know from experience what alcohol and loneliness can do to a person, despite a person's good intentions. If it was a question of just having a friend stay the night...same or opposite sex, I would say that you should try to trust him. But since it's a question of having someone over that acts like more than a friend...I think you should talk to him about it.