WP is probably as safe for a teen/young adult as any other reasonably ran message board for those with AS (not talking about the esoteric ones here). I would just suggest that you keep tabs on your son and recommend that he stays away from the adult forums (although anything said there will more likely be heard in the average US Middle/High School) and possibly The Haven. There are teen forums here. I do not know if Alex ever implemented age controls so you will need to keep an eye out.
As for a teen only board, I think the issue is that we spend so little time in our teen years and they are miserable, as soon as we leave, we never want to look back. Even if a teen created such a board, I would think that as soon as they hit adulthood, they would probably abandon it.