I could've misinterpreted what I read, but something I read said that schizophrenia and autism are opposites of each other, i.e. one has too much dopamine and the other has too little respectively, so I believe this means it's impossible to have both. I also read on the wiki for schizophrenia that schizophrenia isn't diagnosed if a pervasive developmental disorder is present. As for what's going on with your son, I'm not entirely sure. I thought I was really knowledgeable from experience on autism, but then I found out I actually have schizophrenia.
Psychosis is a specific symptom that can have many causes so thats complex. It would be important to speak to a neurologist to see if the seizures he had could have caused any permanent concerns or be part of the same concern and do any tests they believe might be of help such as MRI's. Also see if you can access any past medical records from then. Then from there they could speak to a child psychiatrist and see what the appropriate next step is.
Have you found out anything? My son is 19 also had unexplained seizures, presents himself with schizophrenia symptoms and is autistic. Trying to get answers as well