Nautigal,
That is terrific. We sound like we are in nearly the same place. My DS (3rd grade) goes up to 5th grade right now for math, and we are looking at whether he will bump up to 7th grade math next year (when he is in 4th), or to hold at the two years of acceleration for next year and bump the extra year after that (everyone agrees he won't need both 6th and 7th grade math before Algebra, although I am actually the one who needs convincing - I don't want him to get too far along and then we all realize he doesn't have a truly deep understanding of some important underlying concept and it causes problems later).
My concerns are similar, middle school is a lot more work and they move quickly, without as much hand holding. Not to mention he will have to move between the elementary school and middle school and none of us really know how that will work out just in terms of logistics. And I agree, if he is going to skip a grade (which remains to be seen), I don't want him to miss 5th grade (a big deal at his elementary school).
I am curious about how your school deals with other subjects? Does he have a small group he works with on differentiated work? Is there a GT pullout or class?
