Most Helpful Guy
This is more about a dominance/submission fetish than incest. He doesn't really see you as a daughter, but he wants to role-play a parent/child relationship with you, at least during sex and possibly more. There is a whole genre of erotic romance (or porn) about this sort of role play (Emily Tilton, Carolyn Faulkner are typical authors). There are a fair number of women of all ages who are into "Dominant Daddy / Little Girl (DDLG) play. But if it's not your thing then tell him that it doesn't work for you, that you expect to be treated as an equal adult.
Most Helpful Girl
When two young couples have sex, sometimes they role play and call each other daddy and mama. That does not mean they are related. You are too young to have sex with a man of age 40. He had lot of experience in sex than u do. He had already had sex with womn of different ages. Now that he got tired of all those he want to try new, experiencing new role play with a very young age girl.