AoPS actually has an awful lot of quite young kids. You son might be interested to ponder whether others in his class were being equally mum about their ages, for the same reason he was. (I certainly remember my own DS doing that - but once one kid admitted to being 9, several others did too.)
If your DS is actually feeling like he's freakish for being the only one, it might help him to show that there are many, many like him in AoPS. You may find age-related discussions through a search of the AoPS forums, or even, depending on your privacy concerns, want to post yourself asking other young ones to share experiences. Or send a note to the teacher or to general AoPS staff asking if they could privately connect you to a couple of kids - the people there are really responsive, and I am sure they would totally get your challenge. Your DS shouldn't feel alone - because he is definitely not alone! Sometimes the attempts to camouflage work too well...
