@boomerang,
You're way ahead of me. I would start with the basics and none of that other stuff at his age, while leaving room for his asking - well, I guess I did always do that.. I think I was the main purveyor of both info and sense to my niece from age four on. Luckily, she was generally keen to talk and ask me, ask me, ask me, ask me, ask me.
It's odd and exhilarating, we're peers in talk at this point. She is savvy, observant. I'm, what ere befall, me, been through goofs. Not at all one sided - not that it ever really was, as she was always observant, even at four.
Still, Boom, I'd try to include Dad, somehow, or I think I would. My niece wasn't a boy, so I dunno.