Will your DS be going to the elementary school which houses the gifted magnet? It might not be that bad in a K/1st split classroom. I presume there might be siblings like him.
I have only one hopeful expectation as far as academics go next year. And, that is that I really hope the school make some effort to help DD feel secure about her level of reading--that it is good, ok, normal, not something she needs to hide. Otherwise, the curriculum seems really fun, explorative, and open-ended, even the literacy stuff. Maybe this is a perk of an independent school.
I think peer mentoring should be utilized in all classrooms. Like someone said, it is great for development of Theory of MInd stuff. But, it won't help in a poor school fit. And, it should not be relied upon to ameliorate that.
I have always thought that your DS would be a good candidate for a skip. You know him best. Maybe explore a skip in a private school setting until second grade.