Push, push, and push.
It isn't the subject matter that is upsetting your son at school, it is something about the way they are handling it with your child. It could be the sensory input in the classroom environment; time pressure; any number of things.
If your instincts are telling that they are dropping the ball with him, listen to your instincts.
Other posters have already shared the appropriate legal and advocate information, so I'll add this: If you can't get the school to work with you, seriously consider moving your child to another school. Anyway you can; make it happen; just DO IT. Our kids CAN be happy and successful in school; if your child is not, then something is wrong, and that something is most likely NOT your child. The most wonderful school in the world could turn out to be the wrong school for any one child; you need to find the right fit, and that is going to be unique to the individual.
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Mom to an amazing young adult AS son, plus an also amazing non-AS daughter. Most likely part of the "Broader Autism Phenotype" (some traits).