Hi, MandiK,
I think the above two answers cover the ground pretty well. My youngest was about 5 when he heard the "story" and it went pretty much like Questions related in their post. His brother was 12 and got a bit of a different story and more explaining.
Give him, I'd suggest, the simplest explanation he can handle right now and let him know that if he has questions as he gets older, then he should ask them. I would imagine you've used that with other things he's asked about over the years. It's a good tactic, imo.
Just as a "heads-up" I'd also make it a point to let him know that there are "family matters" that one discusses only at home. My, now, 10 year old just finally started telling his friends without asking about that, "well you said you wanted me to feel comfortable," (true) but found some of his friends rejected him after he's told them. So then we had the "family matters" talk.
Prolly could have saved him a lot of grief had I started with that as well.
Hugs,
Nichole