Hi, op here, I thought you might like an update as my books finally arrived: Who's Got What? and Mummy Laid An Egg.
The Who's Got What one is for very young children, but it was good for them all to read because it named their parts and had a very simple cartoon showing what they look like. We've always used words like bits and willy (don't lecture me, I know how MN feels about those words but as I said, he used to have no idea of appropriate places and times to talk about such things so it was useful back then not to get too excruciatingly technical! I do remember him asking an old lady at the bus stop if she had a willy, that was bad enough) so I read that with them last night.
Then today at bedtime I talked through Mummy Laid an Egg with him, and my youngest wandered upstairs so she got the story too. That was useful because she asked for the whole thing to be repeated afterward, so although he had no questions the first reading, after the second (where he really concentrated) he asked a couple of really astute questions like if girls have a vagina and a uterus the same as ladies, can children get pregnant, and I said no it wasn't possible until they are teenagers (I didn't cover puberty/periods yet) and that children don't come together like the parents in the story did, that it's only for grown ups. So he asked if teenagers could get pregnant and I said yes, so he said "that means that you can't have a relationship til you're an adult then, so you don't have a baby when you're still in high school" and I said there were ways not to get pregnant - he said "surgery?" (I think because we talked about spaying/neutering animals recently) and I very briefly explained about condoms. Phew!
So, still lots to explain but I felt that was as much as needed to be said tonight. He coped well, with only slight red cheeks when I told him they'd be discussing this in school at some point. "Oh come on, that's embarrassing!" He thoroughly enjoyed the book and it's blatant silliness - my children are very silly! Humour is the way forward with them.
Oh god and then I went through it all again with my six year old who has no natural shyness at all yet, she really understood the concept way more than him and was soon yanking pyjamas off trying to have a look at "the baby hole" which she said she thought you grew when you were a grown up. I declined a look, thanks, spoke about bodies being private and belonging just to her, then put her to bed, kissed her goodnight and headed downstairs for a stiff drink. To find there is no booze in the house, of any description. 
Poor planning! For anyone who hasn't done this talk yet, sod the books, you will need at least wine in, if not gin.