Over paid child support be given back to my husband?

My husband was divorced in 2006 and had one child with his ex and two that he loved and adopted of hers. He never missed a child support payment ($908.00) every month for the past 4 and half years for 2 children. After the second child graduates HS the order states that child support will be reduce to $725 for the youngest child. The children were 17, 15, and 13 at the time. Her older child she just gave him to my husband and told him to take the child because he was not good for her. Meaning the child had dropped out of school so she will not receive any child support for him. My husband was in a 15 month tour in Iraq and at that time he always tried to contact the kids but she would not allow them to speak to their father. A few months after returning from his tour my husband received orders to PCS to another state and it made the communication and visitation worst because of distance. Now he retired moved 2 hrs closer to the children, now they are 21, 19 and 17. She does not allow contact with the children, kids already has their mind set at this age. We just found out that my husband has been paying child support for the 19 year old for the past year and a half and this child has not attended school for the past two years. The mother refuses for his father to talk to him and when my husband asks about the child she says ?why you want to talk to him?? He is fine, working and GOING TO SCHOOL? and to make the matters worst the child lives with his mother, and his pregnant girlfriend. She does not know that my husband made a two hour trip and went to the school she claims he is attending and guess what? The child never been registered at this school and the school district printed out a copy of the years the child attended school and the last year he attended was 2008 for the first two months of the school year. Now we were wondering, will my husband get the extra $183.00 that he was paying extra for the past year and a half. We are calculating not the time he time the child dropped out of school because he was still 17years old but starting from the 18th Birthday. We calculated and came out with this figure. $182. X 18months = $3276.00. We sure can use this money for credit cards payments on credit cards she maxed out on his name before leaving him and going with her boyfriend at the time, now her husband. This is another story anyways. We just want it to be fair. We feel that she always had to upper hand, like their divorce, the children, debt ext. My husband got the shortest end of the stick and she does not stop. We are honest loving people we just want it to be fair to the children as well as my husband.