There is a series by Stan and Brenna Jones that teaches sex ed from a Christian perspective starting with three years old and progressing to pre-adolescence, puberty, and teen years.
*Before I was born
*The Story of Me
*What's the big deal--this is the one you would want your son to start with
*Facing the facts
They are excellent, I think. If I remember, most of them are in discussion/story format, but one or two of the books have the nitty gritty in full picture detail, but that is not the full substance of the book, so don't be thrown!
I love the perspective, even if the lay it all out there details are a bit...frank. But it answers the questions and reinforces them with values.
I like to think of it as compared to driving a car or owning a gun, if we simply tell the function to the child without the proper use and rules of the road, we are being irresponsible. So kudos to you for being a parent and adult and teaching your child without letting him figure it all out himself from elsewhere.
~A.