OP, we use a book called, “The Joyful Mysteries of Life” for THE TALK with my homeschooled kids. It is fully Catholic, and a very sweet, gentle introduction. It has several short chapters, and in chapter 5 (which is in the middle of the book, and which only took 20-30 minutes to get to), it briefly describes the mechanics of sex. I believe that the explanation they give is a beautiful start - but the child will probably need to, and want to know more details about how it all works. I do it at age ten, and so far have done it twice - actually my dh read the book with our ds, and I read the book with my dd. My ds’s reaction was, “yeah I kind of figured that’s how it works.” And my dd’s reaction was of total surprise and more questions like “so you only do it once?” Both raised with pretty similar influences, but the ds is more in tuned with the outside world because of his interest in headlines and pop culture (even though he isn’t exposed to a ton of it, he picks it up here and there). It probably makes a difference too, that the physical experiences that boys have as a natural part of growing up, are likely to make them think earlier about the concept of men and women fitting together like a puzzle. Keep in mind that (as I’ve heard, at least), that at 13, your son has probably had plenty of startling erections, and perhaps nocturnal emissions, etc…At the very least, he has probably thought, “gee, I wonder why my body would do that?” and perhaps come up with a logical answer.
As I said, the Joyful Mysteries of Life is a good start, but not the end. The key is to get the conversation started, get the key information out there, and make openings for questions and more discussions. I think that programs like Theology of the Body for middle schoolers might be appropriate as well, but from my understanding, that book doesn’t actually explain the facts of life. Instead it refers to it in the context of morality.