I haven't read all the responses, but "It's Perfectly Normal" is a great one. I think they make a version for under 10, but I'm not positive. The 10 and up book is FANTASTIC.
"Where Did I Come From" was the first book I ever saw on this topic. I haven't looked at it since I was about 10 so I don't know if it's still a good resource, but you might check it out and see if you think it would be right for Petey.
I was given this one too. I vividly remember the smiley sperm wearing a top hat and carrying a rose.