In this dream, pregnancy is symbolic. It represents a new break through in your life. Since you and your ex are communicating and working through your relationship, this is the new break though you both have reached. This is relating to those feelings of peace, as you and him are working out your problems and are coming to a place of peace in your relationship, With you both havng a child together in real life, this is wonderful as it will help in your ongoing relationship with raising your son.
Let me know if I can be of anymore help, and thank you!