Wouldn't it be ironic if he dumped you for MsThang and she ended up dying (not that I would wish that for her) on him? While in the meantime, Armageddon™ keeps postponing and you keep right on living your life to the fullest?
It's pretty sad rationalizing that you should dump someone because they're going to die some day and there are "other fish in the sea". How would he feel if you did that to him?
I would capitalize on the fact that what he's doing - sneaking around behind your back, becoming emotionally involved with someone else, is something the WTS describes as "dealing treacherously with the wife of your youth" (I forget what scripture that is) and is extremely disrespectful of you, of your daughters and most of all toward the Creator in front of whom he vowed to "love, honour and cherish, till death do us part" (whatever the WTS rendition of that is) and the Congregation™/Organization™.
I wonder if he has any Privileges™ that he wants to keep?