I say this because I think these two types are most likely, but I don't know which one he is. I know they're different quadras and all that, but please work with me nonetheless. He's an INFP in MBTT, and a 9w1 in the Enneagram. He wears that expression most of the time.
He's a very friendly person, and a very good conversationalist. He's quick to criticise you if you're wrong or do something he doesn't approve of (I get this most frequently when I'm playing on the Xbox, and he says "you're rubbish" or "you do it like this, not that", and that kind of thing). He says he is disgusted by the human race, and the way in which we live and act towards one another and the world. I don't know what this stems out of, hence why I can only really attribute it to his having a 1 wing, and thus high standards. He's very picky romantically.
He's very procrastinatory, but once he embarks on something, he finishes it. It really is just a matter of getting him to do something. He's quite honest and open too, primarily with the people that he likes. He can often put on a polite front with those he doesn't really like, but he generally steers clear of them, and they of him. I get the feeling that the other guy I posted that you're trying to type may be his conflictor. He has frequently told me that he gets annoyed by him (and his loud, 'annoying' voice), and he annoys him more by the day.
He likes to have fun as a person; he's very sociable and will go out and enjoy himself. He's by no means a loner, and actively makes an effort to engage with people. If we're watching a film or something, and an animal gets annihilated, out come the "awwws", and they are genuine - you can tell that they are. He's not a complete pushover - if you ask him to do something and he can't be arsed doing it (e.g. "will you put that in the bin please mate?" or "get this for me please") he won't do it. He makes me laugh when he gets annoyed or angry, because he doesn't ever unleash full anger; he'll often just literally say "grrr" or whine or whinge in a high voice about something. I take the piss out of him and we have a laugh about it. He takes jokes about himself very well; he's not uptight at all.
Our relationship is fundamentally good. Besides the aforementioned criticism, I have no genuine problems with him, and even the criticism I take with a pinch of salt i.e. it doesn't bother me at all. I just jokingly tell him to fuck off, and he's got no problem with that. We can talk about the Enneagram, MBTT or (as I've just recently introduced him to) socionics, and he's quite interested in it. He doesn't like having to read about it, but he'll happily sit and listen to me (or tell me he's bored if he is). Along with Tom (the guy who I asked you to type in a joint effort with the me description which you didn't know was me), I plan on sharing a house with him next year too. I split my corridor down between those who I could not live with for a year, and those I could easily live with for a year. He I could easily live with for a year.