Thanks for your comments everyone.
We certainly do not plan to discuss any intimate details, no children want/need to know any of that.
The feeling of extended family should be already present since both families are always hanging out together. So, our teen is always exposed to the love and support our quad brings to him.
So, I think we want to do is explain the richness and rewards to our non-monogamous lifestyle and that it is ok to be a little different from other families. We want him to continue to be comfortable around the other adults in our lives whom we love and to feel normal if there is a hug here or a kiss there... similar to what his mom and dad do already from time to time.