Sometimes I get into irritable moods for seemingly no reason. In actuality, I'm pretty sure these "annoyed at the world" moods comes from subtle (or not so subtle), often shifts in mood over a short period of time, from hopeful to disappointed to happy to sad to hopeful and all over the scale. Unfortunately, these irascible moods of mine tend to off-put everyone around me. It's not really that I'm angry, I just don't want to be bothered... Which doesn't really include being alone.
The best thing people can do around me is not really treat me different. My mom will try her hardest to cheer me up, which works the exact opposite of the way she wants it to. My friends will usually tread a little lightly lest they incur my wrath (I'm actually really mean sometimes :blushed: It offsets my normal lovely disposition), but otherwise our interactions are normal. If they're in a kind mood, then they might act a slight bit more silly without the obvious "cheer up!!" message, which works like a charm.
Just hang out with him, do what you normally do, and make sure he knows you're open to talk. Too much digging and prying will lead to badness, usually, though.