I'm going to tell you the unvarnished truth. The result may be an answer that you won't like. It's not an answer that I like either, and I have fought about this type of inequity in court before, only to be told by the judge that "my hands are tied."
Back in 1984, Congress began to make massive changes in what is called the Title IV-D welfare laws, under which the federal government reimburses state governments for public assistance to families with children. As part of this legislation, Congress required that each state implement "uniform guidelines" for child support, which was intended to force many "deadbeat dads" in extremely poor neighborhoods, from having children outside of marriage, and then the mother would seek welfare payments from the government to pay for their care. This system was creating welfare households in which mothers would have as many children as possible in order to generate income without having to work. So, it seemed obvious that a change was needed.
The change required every parent to pay child support according to a formula, and only in extremely narrow circumstances, could a court modify the formula. The legislation was fought over in court for several years, but it withstood almost all attacks, and when the dust settled, the system that we have today was confirmed.
Part of that system is a federal law that unequivocally states that a court has no power to retroactively modify child support. Title 42 U.S.C. 666(a)(9)(C). And, the consequence of that law is that unless a parent asks a court to modify child support during the minority of the child, the court is absolutely powerless to modify it after the fact -- no matter how unfair the law affects the parent.
So, in cases such as you describe, a parent can be stuck with a huge arrears which he will never be able to repay -- simply because the parent was unaware that he needed to ask the court to modify support while the children were still minors.
And, that's where you're at today. Now, if the father decides not to pay, then he can be prosecuted for contempt. At that contempt hearing, the father can explain that the children lived with him for 8-10 years while he continued paying support to the mother -- and the judge could conceivably refuse to find him in contempt, because of the unfairness of the situation. But, the judge could also decide that the father should be punished and that could mean jail time or community service -- and in either case, there would be no reduction in the amount of arrears.
BotXXXXX XXXXXne, the only way to get rid of this sort of debt is to make some deal with the mother to pay a lump sum amount, in return for the mother getting a "money judgment" from the court and then giving the father a "satisfaction of judgment" in exchange for the lump sum payment. That would terminate the arrears obligation for whatever amount is agreed to between the parents. But, if the mother doesn't want to compromise the debt, then the father is stuck -- he will have to keep paying on the arrears forever -- or until paid in full (which is usually forever).
No independent fraud action can be brought to remedy the problem, because federal law preempts any such action.
It's incredibly unfair, but like I said, this is the unvarnished truth, and without that lump-sum payment scenario I described above, there is simply no escape for the father.
Please let me know if I can be of further assistance.