T.S.
Hey,
I personally think 6 is a little young to have that talk. I taught school for about 8 years and mostly taught Kinder through 3rd grade. I think about 3rd grade they are ready for it and can understand it a little more. My daughter who is 7 has asked a lot of those types of questions too lately, I try to tell her at her level what I can. She is more asking like how do babies get out not in. You can always talk about eggs with him and discuss that mommies all have eggs inside that grow into babies. I used that method with my daughter, and she asked if it was like ducks and their eggs. I went on to explain that the ducks eggs come out like eggs while in people they develop into babies. He actually might benefit from picture books of how babies develop and try to stear his questions from how do they get in, to how do they grow.
Good luck. Parenting is so much more than I thought it would be. :)