My DD did AoPS pre-algebra when she was 8 and about to turn 9 - the age difference made no difference to her and she totally rocked the class.

They are very accepting of ALL ages of students and technically students are not supposed to exchange personal information- possibly to stop kids like your son not feeling welcome.

He should not feel unwelcome - he will see that he belongs there if he sticks the temporary feeling weird out...

Become what you are