Please don't make up a story about a stork! If he's curious enough to keep asking, then he's old enough to know the truth. But be very vague. A six year old boy isn't ready for details yet!
And I don't think c-section births sound horrible. All my kids were born via c-section and I've showed them my scar. I told them that they just wouldn't come out so the doctor had to go in and get them! They didn't seem horrified.
A nice way to explain it to little ones is to say that "Daddy planted a seed in my belly and you grew there just like a plant!" Kids see and understand plants, so it's something they can understand. By the way, my kids never asked "How did Daddy plant a seed in your belly?" But if yours asks, you may want to go get a book that explains this, but one designed specifically for younger kids.
After I have "a talk" with any of my kids, I remind them that this isn't something to talk about with their friends, only mom and dad. At least for now.
http://keystosimpleliving.com/kids.php