Legally, there's not much you can do. You set the boundary, he chose to cross it, if nothing else he is telling you even more about himself.
Next time he says divorce him, you could call his bluff. Don't ask if it's what he wants, just say ok. Maybe even do a little research so that you can tell him what you need to get the process started. That will shut him up quickly!
One of stbx's unfair fighting methods was to threaten divorce... the time I finally said ok she backpedalled really fast. She went from being miserably unhappy to being content in about a half second. By taking control, you can disarm him.
