Hi Grinity. That is a really tough one, and I can't exactly speak from experience as a parent, because my oldest is in 1st grade. BUT, as someone who went to a very small private school from K to 8 - I felt very pigeon holed there the entire time. Groups of kids didn't really wildly shake up the cliques after the first couple of grades. I was a very shy kindergartener. I never felt enabled to reinvent myself until high school. So I definitely understand where's he's coming from. But it is also totally true that expectations at that age can be very different from reality, so make sure you lay that out for him as clearly as you can. But I do know for me socially, a bigger school was always a better school. I chose to go to a huge university for that very reason.
Anyway - keep us posted! Good luck.