I bet he'd like you to sign a waiver. Sign nothing until you have spoken to a family lawyer in your state. Although I can't see any advantage for you in signing a waiver there may be additional facts that I don't know here. Also even if you get an order that he pay back child support how likely is it that he will, given his history and the fact that he is now in prison?
Yours is the quintessential fact pattern that supports the notion that child support is a failure as a replacement for welfare.
The original idea for replacing welfare was that poor daddies must be made to support their kiddies so that poor mommies did not have to sit around all day with their kiddies. Presumably money from daddy would allow mommy to go to work while paid daycare was hired.
The federal government added a matching fund for dollars ordered paid. Hence, our courts made the original idea spread across the entire middle class population.
So the well off daddy had to lose his child to mommy in order for the state to take money from daddy and give it to mommy whether mommy needed the money or not. Uncle Sam also decoupled visitation from the requirement to pay child support so a father’s rights industry was spawned almost overnight. Also daddies who fight for more contact with their kiddies face divorce industry experts appointed by a court intended to make daddy lose his fight so the state can get a greater amount of matching funds than had the mommy been the non-custodial parent.
Amid the gaming, the assumption made that loses your case is that daddy can work enough to pay his child support. In your case, that is obviously not possible. Instead, you were filled with high hopes that daddy can lessen the pain of poverty through government coercion: loss of licenses, garnishment of wages and incarceration.
The adage that applies here is you cannot squeeze blood from a stone. If daddy is destined for a life in prison, he will not pay you anything. It appears from these facts that he cannot pay because he genuinely is incapable of participating in the normal market economy. You lose.
The last question is whether you would be so kind as to waive his arrears. Under the Bradley amendment, no court can waive child support arrears, but you can. You can walk into court under a petition to modify support and tell the judge you waive all arrears. The judge will set arrears to zero and his child support hassle will be over from that point on.
It is up to you whether you wish to do him such a favor.