My story is set in an imagined world more-or-less identical to the contemporary U.S. My character wakes up believing that he's a medieval lord. He's forgotten his "real" life, only recognizing his parents and partner because they're the doubles of his medieval self's parents and best friend. The identity swap is actually magic, not mental illness, but magic ostensibly doesn't exist in this world, so his loved ones are seeking a medical explanation.
I've brushed up on different psychiatric conditions with Medline Plus (and Wikipedia), but my character's "symptoms" are just a single, extremely detailed & consistent monothematic delusion + total amnesia when it comes to his "real" life. He communicates coherently, doesn't exhibit odd behaviors, and, while depressed and anxious for obvious reasons, doesn't have anything resembling a mood disorder. He's been screened for trauma or disease that might cause amnesia or psychosis as a secondary symptom. Before this happened, he was in good physical and mental health.
So, when he's forced to visit a psychiatrist, how might she diagnose him? Delusional disorder? A fugue state (even though he hasn't wandered off from home)? Can you be diagnosed with a monothematic delusion independently of any umbrella disorder?
It has occurred to me that, since his story is so pat, she might suspect he's faking it - in which case she might attribute it to a personality disorder.
Thanks to everyone for any help you can offer (and happy New Year!)
ETA: He is magically speaking modern "English" (not really English in this world). His speech is formal but not archaic.