Mood Swings/Hot and Cold

I have an Aquarius guy friend, several years older than me (he's early 40s). Behavior changes over the last several months. I've never been so confused about someone in my life. One day (or week) he's happy, and we'll have a good time talking, or going out, and the next, he's rude, cold, angry. If I ask him what's going on, he gets defensive. He's yelled at me twice. The last time was last week, and he slammed his office door in my face on top of it (he was claiming I, as well as everyone else, want him to fail. I made the mistake of correcting him instead of walking away like I usually do). I know work gets stressful (we work at the same company, not together, though), but that's no reason to get to pissy, throw tantrums, etc., especially with a friend. I'm the only one he does this to, by the way, and I assume it's something deficient in him, and not me. I want to help him, but I don't know how - maybe it's a lost cause...I should just punch him in the face.

Has anyone else dealt with this? It's obviously more an issue of crazy vs. sun sign, but any input would be appreciated.