1) does the discrepancy between his achievement level and his grade level explain the behavior stuff? I'm trying to figure out if fixing the educational issue will also fix the behavior, or if there's more going on.
Yes. :-)
How does he behave at home? Or in other places where he is actively engaged? If he has an attitude at home, perhaps there are other things but my gut instinct, having lived this, is that it's the discepancy between the work he is getting and what he needs.
For reference, my DS8 (also 3rd grade) was not in a gifted program in second grade and hated going to school. He was absolutely miserable, something that he'd never been in school. This year, he is in an HG program and loves going to school again. Based on our experience, I would totally take action NOW at the beginning of the school year.