well not sure how I would react to that kind of ultimatum. it is a tough one for sure...His kids are grown men. or suppose to be anyway...I told my children that use to cause me so much grief when I started dating after a divorce that they would not always be around and I needed to be happy. mind you they were under teen years, but at least it helped. I really don't think they would disown him at least not forever.
the only way I see that happening is if maybe you and him were seeing each other before he was divorced.

good luck with whatever you do hon.