TD, I think this is a tricky one. I suspect that part of the reason your DS was disappointed with Cs is that you were. You said that you weren't, but later said
the marks were C's instead of the A's which we were hoping for
so I think on some level you were disappointed and I'm sure your DS would have noticed being bright.

But I also think this one of the downsides to skipping a grade. Definitely there is the social side - being expected to have social skills beyond the age group plus the point Lulu raised about who he plays with at lunchtime. But also, it is a big change academically for him. Instead of being at the top of the class, he is now with older kids, so is no longer the top of the class. He will have to get used to that and he, and you, lower your expectations. In effect, he has been "promoted" to the point where he is no longer "outstanding" in the group. That was your choice (together with the teachers and your DS I'm sure), and there is not necessarily anything wrong with it, but that is one of the effects of the choice that was made.

Often when it comes to skipping grades, parents are advised against it, as a child who goes up can be a follower, when amongst their age peers they could be a leader. I have no idea of your individual situation so I am in no way saying you did the wrong thing or anything, but I am just pointing out that he needs to understand that his expectations of results, etc will have to adjust to his new circumstance. Perhaps you could try explaining this to him if you haven't already?