I think athletics are one factor. They should not be the most important factor.
More important, I think, is this sentence:
We are really not ready to grade accellorate.
This seems to really be the issue. It sounds to me as if sports are just one more reason that supports your gut reaction. (Though I could always be wrong...)
There are plenty of solutions besides acceleration if you think it's not the right solution for your particular child. Are there other solutions that might work better? Acceleration isn't necessarily right for all HG+ kids.
Even so, I'd suggest considering why you think acceleration is not right for your child. Maybe make out a pros and cons list and weigh the relative importance of each pro and con.
I'm also wondering, if your son is decent-but-not-amazing at sports, is he even likely to make the school teams if he doesn't skip? If he did make them, would he ride the bench? Would *he* care if he missed that, or is it your DH or you who would miss it?
I was a good-but-not amazing athlete myself. I was sometimes a starter, sometimes a sub, sometimes a bench-warmer. Bench-warming is not worth staying in a poor academic fit for, IMHO. It does nothing for one's confidence. The friendships with teammates are good, and it's good for a GT child to have a challenge in life. But bench-warming typically doesn't do much for a person socially, especially not an HG+ kid. Athletes know the "brains" from the "jocks," even when the "brains" also play sports.
I guess I'm saying that I'd recommend thinking over the pros you've listed here re: sports and be sure your child would actually reap those benefits if he didn't skip.
Also, I'd defnitely second the idea of looking at the Iowa Acceleration Scale. It helps with weighing pros and cons.
Also, you should talk with your DYS family consultant about these sorts of issues. They're very good about helping you figure things out, asking useful questions, supplying info for decision-making, etc.
HTH!