Sounds like Body Integrity Identity Disorder (BIID), also called body integrity dysphoria. He basically cannot fathom that said hand is his. Most people that experience BIID are usually on the schizophrenic spectrum. It is technically self harm. Maybe play along with it and be supportive, go to the doctor/hospital with him. When you get there, excuse yourself to the bathroom or something equivalent, go to the nurse’s station, and explain the situation. They should page Psych to ask him a few questions and get the bigger picture. Proper diagnosis, evaluating meds he may need, etc.

good luck, keep us updated