Lots of boys become very anxious about sex. It’s cruel to deny them the knowledge they need.
So, have sex books in the bookcase, where your son has easy access to them. Don’t get stuff that’s too sophisticated, too biological or too abstract. Instead, look for books that get down to basics and explain which bits are which, what they are there for, how to use them, and what kinds of feelings are involved.
On the other hand, don’t force this material onto him. If it’s there in the bookcase, you don’t even need to draw attention to it. He’ll find it when he’s good and ready, and if you have a positive relationship with him, he’ll come and ask the questions when he’s good and ready.
However, if by the age of eleven or twelve he hasn’t asked you anything, it suggests that he’s either uncomfortable with the subject, or uncomfortable asking you, or both. It might be worth trying to set up situations where he can talk to people with whom you know he has an open relationship, like one of his teachers, uncles, grandfathers, or big brothers.