My ex husband is a jerk?

and can't seem to be nice ever! We had agreed on custody and had things ready to settle, now he has decided no he won't settle and wants my son. There is no talking to him at all about anything. He was rude to me and the guardian ad litem. Our divorce was final months ago we have both moved on and I am extremely happy with my fiance. My fiance and I get along so much better than my husband and I did. All my husband and I did was fight and argue. Yes the separation at first was hard then I took a long long look at myself and realized that it was for the best. My ex and his mom are causing nothing but heartache for me and think that my fiance and I fight all the time even though we don't. Yes my son lived with his mom based on an agreement between all parties for him to finish school but that was agreed on by the GAL, him, his mom and me cause we didn't want to change his school mid year. Now that we have gotten him ready for the move they don't want to do it and are fighting me to the point that he was even rude and mean to the GAL. Is there anyone that can help me with the best way to handle this situation? I have bent over backwards to be nice for our childs sake but nothing is helping.