That's a pretty tenacious kid you've got their Otterpop. We talked about this frankly with our son, I think, right around 6. Same kind of situation - trapped in a car with the wife and him pestering us for the details.
Up to this point, we had used "anatomically correct" terminology for the critical body parts involved, so that was helpful. I went the "flower pollinating" analogy route and this worked out well in terms of talking about how pollen gets into the flower, combines with its other half and starts cooking a seed. Only in this case, you were the seed and instead of growing inside a fruit or some such, you were inside mom. When you were ready to "sprout," you came out and starting growing on your own. Or something like that. He was pretty disinterested in the whole emotional element of things and more focused on the "mechanics" if you will. More of an intellectual exercise at this point in their development, I think.
Anyway, it went more smoothly than I would have expected. Occasionally he explains the process out loud to us and that is a little weird to listen through when he is saying words like "penis" and "vagina" (which was much cuter when it came out "pagina" but those days are over). Did I say all that?! Yikes!
But its all good. I would rather have him be informed than not. Not long after this conversation, I overheard him on the playground with some friends (boys and girls) and they were all talking about how people get pregant (must be a developmental thing). Some were saying that "the boys stick their pee pee in your belly button" and someone even said it went into their butt, so I was pleased that he at least had the facts straight...