You get over it by not romanticizing the relationship. Remember all of the reasons why you broke up with him. Just don't remember the situations or circumstances, remeber what they felt like. Reaffirm your decision to break up in your mind. Perhaps he was just coming back and saying all the right things because men hate to loose.
He is moving on. This is great for you. It gives you the opportunity to begin to move on too. We all learn from our past relationships. We learn what we want, what we don't want, how to be, how to not be, what we like and what we don't like. Pray he is always picking women you can respect and like (since he was trying to get back together with you it is likely he will pick someone with his perception of your qualities). Praye he picks women who will be fabulous to your little boy.
Now is the time for you to do things and go places. Explore the world around you. Get your son involved in activities. Go to parks, museums, children's plays, baseball games, swimming, tumbling classes for tikes, etc. Live life to the fullest. What are some of the things you always wanted to do. Maybe now is the time to get them started by branching out of your comfort zone and meeting new people and making new friends. In time you will discover this wonderful new life you have made for yourself and your son and then there will truly be no turning back and health and healing.