I'm not sure I can be helpful, but I did want to chime in. My DS(almost 7) was grade skipped and then further subject accelerated in math. It's a combination that seems to have worked out well, although he's not as advanced as your DS. The subject acceleration just happened so he's at the top-middle of his class for math, but I fully expect him to be at the top of his class in math by the end of the year since he has a good head for concepts. Like you, I think, we don't want to have him skip a grade again since he seems to fit in socially really well with his main class, but we do think it's very important that he's learning something while he's a school 7 hours a day! It's always a work in progress, both with math and with all the other subjects he's way ahead in and for which he is not accelerated. But, for us and for many of us here, it's sometimes about least-worst fit and the best fit we can get for now. The thing we're working on now is getting them to trust that he's ready to work with more advanced concepts rather than their current focus on math fact speed tests.
Are you in a position to maybe arrange for a math major at your nearby university to meet with your son once or twice a week? It may have to be at your expense and after-hours, but it might at least be interesting to him and allow him to explore things more advanced and in a more conceptual way. It might be a long-shot, but perhaps the school would allow him to work on assigments that the university mentor gives him during math time at school, particularly if they have a chance to see his work (he could maybe "turn it in" to them after he's reviewed it with the mentor). I've heard of that working in some circumstances for very advanced kiddos (like your son seems to be).
I wish I could be more helpful.
