I have a a couple manic depressive/bi-polar relatives on my Dad’s side of the family. For two or three of the most serious, until they got help and got on the correct type and level of medication, some of the grandiose thoughts they had sound similar to your nephew. The biggest issue after the medication was correct, was ensuring they stay on it. It seemed like until they learned/accepted that, they would go off their meds, as they felt even better, then go right back into the rabbit hole of schizophrenia and usually had to be committed to a hospital until their medication took effect again.