Well if this is just fantasy and he's not actually pushing for it...leave it alone and chalk it up to simply fantasy that turns his mind on.
If he's pushing for it...nip it. Tell him it's never gonna happen because you know YOU can't handle what it would do to you emotionally and because of that your M couldn't handle it either...then tell him that's the last you want to hear of it.
Sometimes people verbalize their fantasies...just to hear them. So...how's he going about this?
