My nephew is the sweetest nicest little boy on earth. I refused to believe his autism diagnosis as he seemed fine at home. It was only when I volunteered at his kinder I saw it.
Normally he copes fine. But put a lot of stimulation around him (noise people etc) and he melts down.
Keep seeing docs till someone listens. Also ask the teachers if they think he might be special needs, they're pretty good at picking it but often don't feel comfortable bringing it up with the parent.