When I was little, I was always told that my body just didn't make a chemical that it was supposed to make so I had to take it as a pill. My mom explained a little more when I got older and started asking questions, but she really didn't know a whole lot about it. I've quizzed my endo on it the past few times I've seen him, plus I've done a lot of research on my own on the internet. Just tell him the basics for now, and don't dwell on it. He'll want to learn more when he's ready.