I do give you credit for telling him the potential consequences. I didn't get that.
Can you see that since you are heading down that path, you will be doing yourself and your h more of a favor by doing it quickly? And the kids, they will notice every dying and age really doesn't matter. Children that I have seen hardest hit are the older ones.
Now as for your ultimatum, obviously, he doesn't believe it. But whenever you decide it is over, it will be ugly. And you have been the one who said you will walk away, so I ran with that.
One last question - do you want this marriage to succeed?