T.S.
The books my children loved are: "Where did I come from?" and "What's Happening to Me?" by Peter Mayle, and "It's Perfectly Normal" by Robie Harris.
You might just want to mention to the parents that their son is going to get his questions answered one way or another and that if they open up the communication lines they can then be aware of the information that he is getting.
Also, it is a false belief that information is what leads to sex. It has become abundantly clear that a lack of information is what leads to early sexual activity, addiction, pregnancy and the spread of STD's. Only open, clear, direct and continuous communication will allow for youth to make informed choices.