Portia, since my DD is still little, I have no BTDT advice to offer but is it possible that over the years, he has gotten used to the idea that he is the smartest child in the room even if there are slightly older children ?
I have done what I could to shield DD from comments from other adults that are meant to be compliments and we try to focus on efforts and her own long-term goals but still, they are starting to sink into her head. If it goes on for four more years and she suddenly meets classmates who are older and wiser, she might go through a bit of identify crisis. She doesn't mind being a not-so-stellar ballet or gymnastic student because that was never her "thing" but by 1st grade, she might be used to standing out because of her precocious academic skills. I personally think it's a good thing that he is facing this now, rather than later.
