Thanks for all the terrific advice. I'm sitting here making a list of pros/cons and questions.
This is a large school, with TWO gifted classes in each grade except K. They do mix the kids up, but he'd probably be with half his class the following year. It's a neighborhood of high-maintainence mamas, so there might be some talk.
Grinity, you have a good point about school being for academics. I can try to convince him. Ultimately, though, I'm not going to force him into a grade-skip against his will -- and it's one of the IAS disqualifiers if the child doesn't want to be skipped. Especially since he has such a good attitude about skipping at the beginning of next year, I don't think I would ruin that for the benefit of a few months of second-grade work.
I'm going to meet with the principal and teacher and see what we can hammer out.