I think that your idea of discussing at least puberty, if not sex is a sound one. I think tht kids talk about these subjects a lot sooner than we realize and getting the proper information from a parent will remove some of the mystery. It may also open the lines of communication with your son for when he does have questions about sex. I have two girls and when they were younger I always answered their questions as honestly as I could.I remember looking at one of my A&P textbooks as we discussed the menstrual cycle and what was happening in their bodies.The oldest was about 10 at the time. Mine started having sex ed about that age at school as well so it was a good opportunity to talk about what they had learned.Best of luck with your talk.