Congratulations on becoming a grandfather and on raising a daughter willing to handle her responsibilities. You (and your wife) must have done a lot right in the past 16 years. Good for you. Your continuing love and support for your daught will make dealing with the situation better for all involved. The new dad has a bunch of growing up to do. From what you've said he might be willing to do it, he just doesn't know how. Maybe the Army will teach him, if he sticks it out.
fwiw, my step-sister, who is a single mom living on her own, is Valedictorian of her college graduating class.
I'll put you, & your family, on my prayer list.