Some other possibilities that fit the symptoms that you're seeing--
anxiety
sensory processing disorder(s)-- leading to distractibility
asynchrony of some other non-specified type (?)


I guess given that you HAVE seen settings where he did not appear to have ADHD (camps, etc) and that medication trials didn't help-- I'd lean toward assuming that ADHD is not what is going on.


Schrödinger's cat walks into a bar. And doesn't.