To me, it sounds like what he needs to learn is a combination of the kind of arrogance that says "ok, I know this is your opinion, but I'm going to go for it anyway" and the kind of humility that says "all right, this may seem a bit silly to me, but as long as there is a chance I could learn from it, I'm going to do my damnedest to work at it anyway".
I don't think "teacher is a knobhead" really gets you very far along that path. It's what my dad said to me after I had failed an English lit module. It didn't make me try harder, it didn't make me learn more, it just encouraged me to feel sorry for myself. What he should have said was "ok, so where do you go from here?"
In my case, the problem was pretty obvious: I was by far the best student on my course so I hadn't worked hard enough, I didn't think there was anything these people could teach me.
Of course there is no knowing what your ds needs, but there is unlikely to be any problem where sitting down and thinking about how he can improve isn't going to be a solution.
And I am speaking as the parent of the girl who got rejected at the audition of a major drama school for "not having enough movement in the lower part of her body" after she had explained that she had a badly sprained ankle and couldn't put weight on it.
Other health-related problems mean that I am having very similar conversations with dd at this point, though I am not doing much to guide her as she can do that on her own; I'm just there to make encouraging noises. In her case, what she is planning to do is to audition for more practically based courses, and if that fails to take a year out working and do evening courses in acting. She has thought it through seriously, she knows it's not about dreams of stardom but about really wanting to do this kind of work, and she is prepared to let it take the time it takes and risk failing. I could never advise anyone to do that, but I'm not going to advise against it either.
And OP, lots of people get into the big drama schools who are not "posh"- wiry-haired northerners do actually get in.