How To Pay Back Child Support When In Jail Or Prison

I am a friend of a person in federal prison. His wife divorced him after he was sentenced. He has 3 children. He receives a statement from child support enforcement every 1 or 2 years about how much back child support he owes. He has every intention of paying child support, but it is already going to be hard to find employment based on his record alone and he already has a garnishment in place. Is there anything he can do to reduce what he owes or what kind of legal rights does he have on the amount he has to pay back? I do want to add his 2 oldest children are both grown now and have children of their own. There is no doubt that he owes child support, we are just concerned about how hard it will be to pay it back.