You seem to have good guesses on it. It is possible to have schizophrenia and dissociation. I used to hear voices in psychosis (Auditory) but they are different from dissociative voices (More mental). My dissociation was caused by use of psychedelics and is treated with olanzepine.

I don't think most psychiatrists/psychologists are trained much about dissociation.

I think the important thing is to make sure that your son is able to distinguish dissociative symptoms from those of schizophrenia (Because they can overlap) - So he doesn't get hospitalized for dissociative symptoms that can be treated with therapy rather than being hospitalized.