Originally Posted by galun
We are acting on what he wants...
While it is very important that you and your son are in agreement on what you are advocating for, when advocating you might wish to avoid the word "wants." For example one might rephrase this as... "We are trying to best meet his needs." Keep in mind the needs of a child are: appropriate challenge, and academic/intellectual peers. (See links to articles in post upthread.)

Originally Posted by galun
we would prefer that he participate in a class (even though it may be only mildly challenging) for the social interaction, instead of the more rigorous AOPS program.
When advocating, you might rephrase "we would prefer..." to "we believe it will best meet his needs..." The proposed math SSA is a possible solution which you have identified to meet his needs for appropriate challenge and academic/intellectual peers.

Originally Posted by galun
If he can get challenged in school, we would remove math from after school to give him more free time to do whatever he wants. Right now we feel that we have to enrich after school.
In advocacy-speak, avoiding the word "wants", one might say: If school can meet his needs for appropriate challenge and academic/intellectual peers, then his out-of-school time can be used to pursue his other interests so he can develop into a well-rounded individual.

Two caveats:
1-Beware the different definitions of "challenge", and do not be shy to explore and clarify meanings when advocating and working with the school.
2-The math SSA may not fully meet his needs, but it may be a strong step in the right direction. On the other hand, I also value BenjaminL's thoughts... there are multiple ways in which to go about meeting a child's needs. AoPS may better meet your son's academic/intellectual needs for appropriate challenge... but your son's needs for academic/intellectual peers may be totally unaddressed... and after-schooling AoPS may also deprive him of the opportunity to pursue other interests and develop into a well-rounded individual. There is good and bad in everything, no solution is perfect, choices involve trade-offs (opportunity cost), and plans can be adjusted as needs change.