I want to be very careful not to come across as not believing your DC, but I do feel that it is worth asking the question of how you know that he is actually being teased as much as he thinks he is. (And it may be that it is every bit as much as he perceives.) Often, kids on the spectrum have a hard time accurately reading social interactions (as you know), so it is not uncommon for them to perceive interactions that are within normal limits for positive or neutral middle school social encounters as teasing, or to lack the skills to shut down smaller-scale teasing before it turns into larger-scale teasing. It might be helpful to see if the school can assign a school adjustment counselor, social worker, or psychologist to process some of these social interactions with him in the moment (social autopsies), preferably with input from adult observers, who may be able to take a bigger picture view of the sequence of interactions.
It would be particularly helpful if a skilled observer could sit in discreetly on his class for a few sessions, at various times of day, and in various groupings, to see what actually happens. (It's true that the kids will be exceptionally good at the beginning of the observation, but my experience is that this age group will revert to type within the first 15-20 minutes of the observation, so a 30-60 minute observation will probably be long enough to capture some of their typical behaviors. Preferably, as I mentioned, on several different occasions.) This would help pin down some of the ambiguity of the nature of the teasing he reports, and give staff some places to target for moving the behavior of the other children in a healthier direction, as well as provide feedback to the facilitator of his social skills group on skill-building that would help him navigate social situations more effectively.
