With limited information, it could just be a fantasy/fetish. Much like the Daddy/Daughter fantasy. There's lots of those videos out there that aren't real, but roleplaying and that may be what he's into.

I would surmise that it would help for you and him to talk through it for your own comfort level. Communication is important at all levels of a relationship and communication about sexuality is of utmost importance so if he enjoys those mommy/daughter fantasies, it would help for you to know about them. If he's really into the real thing ... well ... that's another issue altogether.