No concrete suggestions, but we've been there. I agree that the problem is likely to be something unseen that is taking up his capacity to just deal with the world, rather than the incidents he's exploding over.
For whatever reason, I've noticed that around 9 (fourth when I was a kid, 3rd grade now) gifted kids seem to explode. Of the kids in my family, all but one had a really rough time that year, and the last was 2E enough that he always had a rough time. My mother and grandmother both report really terrible turning-9 years, where various problems required a dramatic change of scene - moving, changing schools, etc. In our private school, the younger kids are there as a preventative measure, but many 8 and 9's come because they had major problems.
My personal, completely unscientific explanation is that the social scene changes enough around that age that being intellectually different from your classmates seems like a less ignorable, temporary, and excusable condition. For me and my daughter, finding GT peers helped a lot. Just knowing that they exist and that we're part of a group helps even when they aren't around all the time. I think we also both felt ignored by the teachers before, and encountering a class where we weren't neglected in favor of the people who aren't able to keep up helped. For me, at least, it was less about the level than about being seen for who I was. My favorite teacher in the primary grades was one who privately acknowledged to me that she knew where I was but couldn't teach me more than the class. She reassured me with looks and the occasional comment that I wasn't completely off her radar.
So- with my family history, I would be looking for a chance to mingle socially with some other gifted kids. Along with dealing with the explosion of the moment, of course.
