Child support modification

My ex husband filed for a child support modification in 2015 but before we went to court he entered back in rehab. Now his attorney is trying to go back and say his child support is a lower amount from the date he filed the modification. Keep in mind we have not gone to court over this and it has not been agreed upon. Can they go back a year and lower his payment amount without having gone to court?
We had child support set during our divorce which was 7 years ago. I have that amount through support enforcement and it has not changed. Does the modification have to be signed by me and the judge?