I just asked him what he thought when he asked me. From there I sort of guided him into it, he got that it takes a mummy and a daddy, and that mummies have the eggs, and daddies have the sperm, and that the baby grows inside the mummy in her womb.
But he still didn't understand the how of it and wanted to know. So I did a pointing finger and a circle finger on opposite hands, and said here's the daddy, he has the sperm, and here's the mummy, she has the egg and the womb for the baby to grow in. How do you think the sperm might get to the egg?
He saw immediately that the man put the sperm into a lady through his willy. I told him that is what we call sex. And he said Ooooo! that makes so much more sense.
And then I gave him the whole, sex is something grown ups can do, it feels nice, so sometimes when they love each other very much they could do it to make a baby, or they could use things that stop the egg and sperm from meeting, and just have sex as a way of loving each other and making each other feel nice.
He was happy with that and went back to playing on his Xbox.
I'm never too sure I get things right though, I have Aspergers and sometimes I am too blunt with information with the kids, because I tell them the practicalities of love, sex, marriage, masturbation, gay relationships, anything they are starting to become aware of and ask for clarification of. I sort of make it scientific and I know I don't explain the emotional part very well.
But I know my son is smart and might work things out very quickly himself (he reads a lot of factual books about anatomy/science/encyclopedic), and that telling him an untruth about things would be discovered, and I don't want them to feel like they can't ask these questions and get an honest answer. My younger daughter gets a less descriptive explanation, that is honest, but not inappropriate for her age.