What would you do if you were in my situation?

You need to sit him down and tell him that you don't appreciate the fact that he's talking to her still and that she's too young and that you're NOT going to put up with it. Ask him what he wants to do. Does he want you or not? Discuss all of these things with him. IF he chooses to be with you tell him that you will not tolerate him talking to her EVER again. If he acts stupid (you know how men can be) tell him that it's not fair to your relationship and those are your conditions for staying with him. >>>