Your roommate is really the one who should tell him to knock it off; he's her boyfriend. This is a problem she should be handling, not you. She's probably scared of the guy, and who can blame her? He sounds like a nutjob.
If she won't stand up to her boyfriend on your behalf, then you have every right to do so. Whether or not it will offend her is irrelevant at this point. Tell her ahead of time that you intend to say something to him about it if she won't. If she doesn't like it, she can do it herself, otherwise you will.
Whom you invite into YOUR HOME is none of his business--and I would tell him so. Just because it happens to be his girlfriend's place too doesn't give him the right to bitch at you because he's psychotically jealous of his girlfriend. That's their issue, not yours; tell him to discuss it with her if he distrusts her that much, but you refuse to plan your social life around his insecurities.
Maybe I watch too many crime shows, but I think you should consider moving out, too. I know she's your best friend, but if this dude is as wacko as he sounds, I would not feel safe living there. He wouldn't be the first insanely jealous boyfriend to show up at his girlfriend's apartment one night with a shotgun and kill her, her roommate(s), and anyone else with the bad luck to be there. I don't mean to scare you (much), but it is something to consider.
Thanks for the question.
I would move out as soon as you are able. Yes, it's a hassle and a pain in the ass. But NO ONE should make you (or your guests) feel uncomfortable in your own space.
Call him out on it, if you think that would help, but I'm like Cary. This loser sounds like bad news. Better to just remove yourself from the situation, rather than make it worse.
once you start using the word psychotic to describe someone, my experience is the further and faster you move away from them the better. Cary is spot on here! his behavior is just waiting to be okay'd and normalized so he can move on to his next level of manipulation and take this further. which he will if this continues without a strongly worded "this is not okay" speech. with somebody, whoever you choose, but you should start that talk...no reason to wait.