I'm a bit like Thunder and redpepper too. When I have something on my mind, I need to process it by being alone. Sometimes, just seeing someone else takes me out of my "processing the problem" state of mind and just irritates and angers me, so I isolate myself and don't talk.
If someone talks to me I don't respond, because that would interrupt my processing. And it's important to me to deal with it so that I can know what the problem is and how I feel about it. Then I can talk about it.
It doesn't happen to me as often as it seems to happen to Thunder, but I can relate to what you say. I'm not sure what the solution is if it's become a problem. I think in my case my main problem would be that people won't leave me alone when I need to be alone (if I were Thunder). And if you do leave him alone, well, then what's the problem?
I'm not sure he can change if that's the way he works. And if he can change, I don't think that change can be brought by other people. It seems to me he's introverted and in the middle of six other people, that might be overwhelming him...
I too would be interested to hear his side of the story.