L.R.
Well, appendicitis does not always have the classic symptoms of swollen appendix -- my child had it and the appendix never swelled at all before they yanked it out. How long has passed? It could be that he does have an infection that is short of full-on appendicitis that would burst the appendix.
Has he been tested for: Diverticulitis? Crohn's disease? Celiac disease? Food allergies? I assume that all the scans ruled out a twisted bowel, right?
More troubling, have you been in touch with his psychiatrist if he has one? And I hope he has one, and sees one religiously, if he was diagnosed with psychosis -- that is very serious and requires serious treatment; is he getting that? If this is "all in his head" it is not a kid being bad, it is part of his mental illness, and requires treatment and not punishment. It's very troubling that he has this psychiatric diagnosis; a pediatric psychiatrist needs to see him in the hospital, now, to see whether there is a psychiatric component to this illness. He's still sick, even if this is in his mind and not his body. A kid who was just "playing you" to get out of school or get attention would not end up going this far-- that kind of kid would be scared to death to go to the hospital. So there's more going on with your son.
You mention punishing him if he is "just wasting my time for attention" but you also mention a diagnosis of psychosis. Which is it? Do you see him as just playing you (and he's a world-class actor if that's the case, which is hard to believe) or do you, his own mom, believe that he may be either really physically ill or really mentally ill? Please ramp down the "I will get you if you turn out not to be sick!" feelings. Please.
Please insist that first thing tomorrow his own psychiatrist or counselor see him or that a pediatric psychiatrist be brought in to examine him to see if there is a psychosomatic illness going on. Your pregnancy could have trigged in him a need for attention that is way beyond what any kid usually experiences. Or...he could be truly physically ill and the doctors are not doing a good job of diagnosis. You need to be more assertive. And you need to believe your child because a kid who is just messing with you for attention does not usually end up in a hospital. Whether the pain is physical or mental, he is experiencing it as pain. Please don't let the doctors tell you what to do entirely -- make some demands, and especially ask for mental health help for him.