[asveikau]

A lot of what you say, I read it and say it's actually very consistent with him believing it.

People who suffer delusions often aren't stupid or even unconvincing. A lot of that intellectual effort you're describing, piecing together plausible details to justify their falsehood, can go into it. Someone I know who suffers these issues had a reputation in the family for being creative, a good storyteller... The hospitalizations and psychiatric diagnoses came later.