I have had similar experiences, but it tends to be when I am trying to make a character do something he just doesn't want to do. When a character goes dark like that, I step back and think hard about what I'm asking of him: is it right for the story or not? The characters often know best.

Now, if this is a brand-new character (which it sounds like, since you don't know his backstory or anything) maybe the problem is that there really is no character there. You are telling yourself that you need this character to fill a certain role (I do hope he's not the protagonist) but perhaps you don't -- or perhaps you need someone totally different to play that story role. Try switching the gender. Or try telling the scene from a totally different POV and get someone else's perspective on what's going on AND their thoughts on that character. Or try writing a different scene from later in the story that involves that character. Maybe it's just that scene that's wrong, not the character.

Maruutsu's advice to put him in a difficult situation is a good one. A person's character really comes out in a moment of conflict. So if you have a scene planned for that character in distress, maybe you can write that scene first.

This is NaNo, so it's not really the time to take a few days doing a character bio or some free-writing about that character. But on the bright side, it's NaNo, so it's OK if your characters are a little catatonic at times.