I was thinking along similar lines! You say he has not been overtly causing any trouble. Has your DS ever mentioned issues like this, or hypersensitive hearing, or any other issue like that? It sounds as if there may be a bona-fide issue. Would the school consider allowing your DS to go to the Library, or wherever, during large assemblies?
I don't see why the assembly would have to be so mandatory. It's not like he is missing an exam in Calculus. And, this would solve the problem of him calling you and getting into trouble. Sounds like a simple solution.
Instead of getting angry and letting him know that he has lost his mind, I think I would try having a straitforward discussion with him. Try to sort thru what happened.
Most kids are happy to get out of academics for something like this.