Not sure that this will help you any but here goes. My wife and I have decided that instead on letting that whole area of life go until they get to "that age", we would instead try to not hide anything from them and answer questions as they come. Now of course we are not graphic seeing as my children right now are 5 and 2 (both girls mind you) but my 5 year old is familiar with the names of the genital areas of both male and female. I am sure it will be different with a boy (my son is due any day) and when that first question about babies and "what are they doing on TV dad" comes from his mouth, I hope to do the same with him.
So anyways, I guess I would say to just be honest and don't get too cutesy with terms. Call everything what it is and explain to him that he will have to be responsible for certain actions taken. Definately explain that there are nasty infections and everything else out there. I would just try to guide him into understanding boys and girls are different and always ask questions about anything he is not sure about. Oh yeah, and be ready to answer some personal questions as kids tend to always want to compare answer to mommy and daddy. So if you answer does not add up, it just makes for more explaining later.
I am not a professional by any means and I will have to ask for help myself I am sure when they get to that age, but I hope this is just going to help when it finally gets to that time in their lives.