Social Security Disability Back Pay

My question is my son's father was court ordered to pay support,he became disable and for almost 2 years i have been supporting our son 100%.in 2010 he was approved for ssdi,his father got a back pay check as well as our son.at this time he was almost 7000 behind in child support i was told that our son would get a back pay as well as father but that the arrears would come out of his fathers back pay to cover the arrears.well went to court and his lawyer ask the court to use our sons money as payment for the arrears his father owed,and to mark his arrears paid in full.how come the father can keep his back pay check but our son had to use his to pay fathers arrears for him.for 19 months i have lived from paycheck to paycheck and had to get help from neighbors,school and family to help keep a roof over our head.

It got to the point i could not even buy a roll of toilet tissue ,washing powders,shampoo and the church had to pay my light bill because it was taking 2 of my checks to pay the rent each month and the other 2 to try to keep light bill paid,get food,pay after school care ,gas to get to work.the lawyer said he has really over payed me . i m lost the court are using sons back pay as payment for arrears his father owed i don't understand this.our son is a special ed student and has a iep in school and is adhd. i have had to use of of my saved resources just to make it.his father has not had one thing to do with our son he was born,i have never ask for anything from his father as i know i would never get any help from him.if it wasn't ordered for him to pay i would not have gotten a dime out of him.if he was $7000 behind in arrears how is it possible he could spend $9000 for lawyers but not pay the arrears?i have a chance to appeal but not sure if i could even win .so therefore the check that ss had took out for me to start with is going back to him instead its not about the money here it is what is right and fair to the child.