How do I talk to an eight year old boy about ?

He turned eight two weeks ago. So he's seen some small parts on television and in movies and he just umes "" is a girl and boy laying in bed naked together kissing. Recently his father had a talk with him about how girls and boys get different "feelings" sometimes after he said he sometimes gets wierd pee-pee feeling. We've really never talked about it and he only sometimes shows curiousity beacause he already thinks he knows what it is. He thinks girls just get pregnant when they feel like it. I'm just wondering how I'm supposed to go about explaining everything to him and when.