Hi WVgal -
and welcome to SR. You'll find tons of advice and friendship here.
About your question -
I hAd to explain 'death' to a four year old when my mother died.
Because he couldn't understand why his grandma hadn't come back home yet.
I don't think I did that well, I remember him crying,
but it wasn't a guilty cry or anyting
it was a 'I'm gonna miss her' cry.
He accepted that she was not able to come back.
Similar, but I understand the difficulty.
I learned something very important at an early age as a teenage parent.
Keep it very short.
"Put it on a bumper sticker' kind of thing.
I don't think you're going to have to do that much delving into it
considering your son wouldn't go with him.
Sounds to me like your son knows something is dangerously wrong.
Are you in Al-anon? do you receive 3-D support somehow?
Someone will come along soon who has had more direct experience with this ...
again - welcome to SR!