There’s a series of books by Robie H. Harris that explain biology and sexuality for kids in very frank, plain language. They’re excellent especially because it doesn’t sugar-coat things or use euphemisms.
For his age the title is It’s Perfectly Normal (Wikipedia).
It talks about AIDS, abortion, homosexuality and, I think, even anal sex in a way that is plain and factual. Read it by yourself, first, so you’re prepared for what’s inside.
I highly recommend that you read it together. It might be awkward but it’s good to demystify and de-stigmatize these subjects. It may help him be willing to talk to you about these issues as they come up.