Hi B.,
I'm the mom of two girls .. 16 and 12.
My 12 y/o started Kindergarten at the age of 4, just at the cut-off date, because she was very bright and bored in pre-K She has always done amazingly well academically, and is frequently bored with grade-level work, She gets enrichment work and is enrolled in the gifted program. If we could afford it, she'd be going to private school.
However, she has no friends. She is socially immature, not only because she is somewhat younger that the other kids in her class, but also because it seems that while she has over-developed academically it has somehow sapped her social skills. They have not kept up with her age. She gets along with adults and HS aged girls and she has a great rapport with kids 1-2 years younger than herself. But with her peers she is just a "brain with feet" It hurts her terribly. She is stuck in a no-mans' land. Smarter and yet more immature than the kids around her. Finishing up 7th grade isn't easy for any girl, and is miserable for her.
In short, it's not necessarily a bad thing to consider holding him back. It could increase his chances of being at the top of his class all through school, and that's quite important when looking toward college. And you might give him time to catch up socially with other kids, which would be a gift you can't imagine at this point.