I agree with what has been said already--that he might not miss much depending on what is in the 7th grade math. I would look at the 7th grade math textbook and see what your son would miss if he skips and go from there.
Socially it might be very tricky at this age. My DS13 had a very bad experience in our public middle school even though he didn't skip grades and for math, his acceleration was done with a few other kids so at least in the classroom there are a few kids roughly his age. My son also goes to some classes outside of school where classmates are mostly high school juniors and seniors. He doesn't really make friends with them but those kids are older and more mature so there isn't any problem either. Middle school is a tricky time though.
