"I dunno, my parents and the teacher thought it was the right thing to do. How 'bout them Red Sox?"
First, remember that the other kids may not be paying all that much attention. It's not a dirty secret, but he shouldn't raise it if they don't. If they do, answer all questions factually, but don't answer any questions that aren't asked. Use a tone of voice which suggests that this is basically a not very interesting topic of conversation. Not that he actively "doesn't want to talk about it," but just that he doesn't himself think it's particularly worth thinking about. Grownups do what grownups do for reasons that often make no sense to kids, so who cares?
This is generally the approach I suggest, and it's generally effective. Usually it's not the other kids, but the accelerated kid himself, who is so afraid of drawing unwelcome attention that he, well, blurts things out and draws unwelcome attention. And once you've done that, it can be hard to go back.
Let him tell you exactly what he's worried about other kids asking, and develop scripts with him for how he can reply in a way that deflects the interest without lying or implying that he doesn't want the more interesting work.
