I don't know the answer to your question. I wonder why first borns feel the need to boss everyone around. I'm sure than not every first born applies here, but one woman I know STILL bosses her siblings and family members around. She has taught her kids to boss the cousins around too. They are grown ups now and if the cousins make a mistake, there is hell to pay.
I don't know - if it were my sister, we'd be either doing battles with each other or not speaking.
I think there's a fine line between squashing a kid's natural leadership qualities and preventing them from growing up to be on the list of the worst bosses. It seems that those kind of people either never got enough compliments when they were children, so that they expect others to tell them how wonderful they are all the time, or to make people do what they say, in order to make them feel good about themselves, OR perhaps they were judged so harshly as a child, that they are paying back the favor to everyone around them.
Of course, I'm not talking about your son. It doesn't sound like the older kids pester him or goad him into the funk he gets into when they show up. Perhaps because they are so much older, he doesn't want to accept them as older siblings.
I would say that he is more of the older sibling type personality than middle child personality, simply because the olders are so far ahead that they don't really count as older sibs in this regard. Maybe he feels that he has to be top dog and they are getting in the way of that.
Dawn