This really is more of a family law question that an SS question. From what you are describing it sounds like your husband only owes arrears but it's not too clear from your question. If your husband is under a current support order, ho could file a complaint for modification, asking the court to change his obligation based on his circumstances. However, if we are only talking about arrears, he will not be able to ask the court to change anything. The only time I have seen significant arrears is when the payor comes into money, such as a personal injury settlement or workers' compensation settlement, and even then I see the interest and penalties waived but not the underlying debt. I would suggest you seek an opinion from an attorney in your area as the above is very general based on general legal concepts and may be different depending on your state laws.
This is a brief response to your question viewing the question broadly. It is not and cannot be an exhaustive answer:
You may be able to settle the entire amount in arrears with a bulk payment for less than is currently owed. I understand that coming up with a large amount of money is difficult especially considering the disability and tax withholding. You need to determine when the past due support was initially calculated (e.g. his son was 15 when a child support order was entered and they looked back a specified number of years to determine arrears/past due support obligations) and how it was calculated (what income did they base the obligation amount on and what other children, if any, were considered in the calculation). There is a fair likelihood the initial amount was not calculated properly.
One would then get into proper calculation of the present obligation (now past obligation) beginning with the support obligation due when his son was 15, then when your husband went to prison, and when he got out. I assume his disability was subject to a wage withholding order prior to and after his incarceration???
After understanding what is owed, what should be owed, and whether those figures are equal, you and your husband can determine the best course of action. Should you try to work through the OAG's office to settle with mom or hire an attorney to get the numbers straight, fight for you, apply some pressure, and potentially achieve a settlement much lower than you could alone??? The cost of a lawyer may outweigh the potential savings. If you seek professional legal help be sure to address the value of the lawyer's service with the lawyer.
Prior to law school, I worked at the AG's office as a Child Support Enforcement Officer. In that position I represented the child and I took this seriously. I had empathy for the parent under difficult circumstances but you won't find much of that. I would suggest contacting a few lawyers offering free consultations.