I believe I'd tell him that if he wants out of the relationship, that's up to him.... but you'll be giving him FULL custody of the kids, because, well, you've got a lot of work to do in your life, too.
Or, he can be a grown-up and go to counselling with you.
Because #3, he goes back to being single and carefree, isn't an available option.
Really, I don't think it's so uncommon for people, men especially, to hit a patch where they think "This is so NOT what I signed up for!" and want out. You just can't go backwards, so you've gotta go forwards.