I suppose, to a great extent, the "right" answer for your family will depend on how you expect your son to react, as well as the moral flavour of your beliefs.
I may be in the minority, but I don't see anything particularly awkward about teaching a young child about healthy sexuality. Rather, I see sexuality as something to be openly celebrated. You have the power as a parent to imprint a positive sexual identity in your child! At 6, children are at an age where their judgment isn't clouded by hormones, and peer influence is still pretty minimal,'so they can begin to appreciate what sexuality means in an agape sort of framework, rather than seeing sexuality trough an eros lens from outside sources. Maybe start with information on a cellular level and work up from there, or discuss the mechanics briefly but center the discussion on love?
As to abortion, murder, rape, etc, I believe treatment of those topics really boils down to your family's belief system. My personal approach would probably link to a discussion of free will and the basis of our faith to provide a sense of the genesis of aberrant human behaviour. I don't know that I would feel particularly compelled to address those issues head-on specifically at 6 unless my child showed a disproportionate interest in them. Rather, I'd approach the positive side of each (respect for life, consensual behaviour, love of neighbour) to give him the logical and moral basis to evaluate those questions maturely if he were suddenly confronted with them by an outside source. Effectively, answering those questions in everything but name to preserve some innocence.
Thanks for bringing your question here. It gave me pause to think through my answer, and I hope it gives you some food for thought.
