It is only human nature to worry.........I wish I could tell you everything will be OK and he will stick by your side. Though the reality is anyone can change their minds. In the end it is a choice he must make. Being calm, rational and understanding will help more then getting stressed and arguing with him.
Let him know you care for him and hope that what you have means enough to him that he is willing to see this relationship out instead of jumping back with his ex. The fact of the matter is you both jumped into this relationship while you were with other people. Even if you were unhappy in your relationships jumping into a new one is going to have its ups and downs.
I am sure you have heard the term rebound before.......while some rebounds end badly others can last forever. The fact is when you end a relationship with someone such as your ex husband you must give yourself time to get used to being single again before jumping headlong into a new relationship. Since neither one of you went this route, little issues like this are going to pop up. Had you waited these issues would have come and gone and you both would not have any old baggage that cause stress in the relationship.
Based on what you have told me, it sounds like he is being responsible enough to talk to his ex instead of ignoring her. By doing so he can end the issues without it hanging over your heads.........which is a excellent idea. So please do not think this is a automatic end to the relationship. If he ignored talking to her then I would be far more concerned that he was not over the relationship. When we avoid old relationships more times then not it is so you can try and force yourself to get over that person. Which often does not happen. Since he is taking the steps to try and talk to her, and end it completely this is a good sign for your new relationship.
I wish you the best, XXXXX XXXXX you have any other questions please let me know.