Back child support question

So my husband has a 9 year old little brother. His parents officially split up 5 years ago. Every since his dad has paid his mom 200 a week in child support not court ordered on top of that he has paid for every thing he needs for sports school clothes christmas and school extracurriculars. He finally moved on 6 months ago. He told my husbands mom that he needed to drop the child support to 140 a week due to bills piling up. She got mad went off on his new gf. Long story short things got ugly she quit letting there son see him when for the past 5 years hes had him every weekend. Well now they went to court for child support which hes fine with its gonna be 110 a week well then she filed back child support and since he has no proof he paid her money they want him to pay 9000 in back child support. Well he said no and she said no because that amount is minimum wage for the past 5 years and she wants the 20 bucks an hr hes made which would bring it to 40000 in back child support. So they are going to court cause she wants the back child support raised he wants it dropped because hes been paying her money. My question is how do we help him get it dropped its totally unfair for him to have to pay this. My mother in law left him and got in a relationship and hes even paid there rent a few times when they came up short and hes paid her car insurance this whole time. Hes a really great person just trying to finally move on after 27 years and now since hes got someone shes not ok with it and is trying every thing to hurt him. Does any one have any advice?