Thanks for everybody's input. I particularly liked your comments, kathleen'smum:
Whether she reaches the highest of heights or coasts at her own speed...
He does indeed seem to be "coasting". He is not particularly interested in doing things, picking up books, reading (which is all I EVER did as a kid), or doing projects. He mostly wants to drive race cars around the house. I agree that I have a lot of preconceived notions about "gifted" that probably mostly come from the way the kids in my generation were -- all four of us had pretty similar profound interests in reading early and often, and I just don't see that in him. At the same age, my younger brother was going to be a paleontologist and knew every dinosaur and the difference between the Jurassic, paleolithic, eocene, etc. My son dabbles in reading about science but doesn't have that drive to read nonstop. He occasionally says stuff that amazes me though, so maybe he's just a lower key kind of kid than we all were.
Thanks again for the thoughtful comments.