If he has a sibling with AS, you need to look very carefully into whether he has similar difficulties.
When my DD1 was diagnosed I felt very reassured by the fact that my DD2 was different from her in so many ways. I was wrong to be reassured. It turned out that she also has AS but it presents in a different way.
I would spend some time doing an quiet activity together, shoulder to shoulder to remove the need for eye-contact and talk through exactly how he feels at different times of the school day.
Think about sensory processing in terms of unexpected touches, bright lights, smells, loud, busy classrooms.
Also the social environment. Is he coping but working very hard in order to do so? What strategies is he using to fit it?
Is he struggling with executive function, e.g. planning complex tasks, following list of instructions.
Does he have problems with central coherence, perhaps knowing which information to include in what he is writing, even though he understands it all?
He may look and behave fine in school, but is he masking?