My son has Aspergers (which I didn't realise at all until he was 9), as part of his Apsergers he has all sorts of sensory issues. Which again I had no real idea about (noise, clothes, smells, touch) - they weren't so extreme that I noticed them, until I started looking for them IYSWIM.
My son has benefited enormously from Auditory Integration Training. Because it cured his hypersensitive hearing - which I didn't even know he had.
I have no idea if your son has this or not. IME it was impossible to tell. There was no real way I could have known that it was the noise of the class which was bothering him so much. Nor could I have known that AIT would have made such a significant difference to his life.
But if your child does complain that the class is noisy, it is possible that hypersensitive hearing is his problem.
(Only possible. It's also possible it's absolutely nothing...)
A sensory trained OT could also assess your son and find out if he has any sensory problems....
But this could all be just me because of my experiences, and totally inappropriate for your son....