My bf is too innocent

Okay so my bf and I are 15 (you might think its too young for sex but on my end its too late for that). When a guy is 15 you expect them to be jacking off often and watching porn or making sex jokes or something. Well my bf has never been told anything. He doesnt even know what sex is, he's just been taught that its something you should be ashamed of. He has started to ask me questions like why do we grind when we make out and such. I want to explain all of these things to him as im very open about the human body and sexuality in general. I just could really use some advice on where to start. Please help!