The Child Support Enforcement Office has no authority to modify a court ordered support obligation. They can perform a review, which, if contested, may go before a court on a motion. However, that rarely occurs upon the request of an obligor.
In Minnesota, any child support arrears are usually taken out at a rate of 20% of the existing child suport obligation, In other words, he would owe the child support amount ordered PLUS 20% of that figure. I am not sure how they do it in ND, but it is likely similar