I'm a little confused as to where the order originates from; I'll assume California for purposes of this post.
I'm going to respectfully disagree with some of the prior posts.
First, the assignment of your rights to child support to the state, which occurs as a matter of law, ONLY occurs on the receipt of CASH aid (TANF, formerlly, AFDC.) There is no assignmnet on receipt of food stamps nor medical services. If there was cash aid received, you can explain to him that the reason you have no 'control' over those arrears is because you do not have a right to them, since they have been assigned. (However, this is actually a very complicated area of the law and there are many variations of assignment, temporary, permanent, partial, etc. - based mostly on when the aid was paid and the federal and state stautory schemes then in effect. What I'm trying to say is that if cash aid was paid, depending on your 'on' and 'off' dates, you may have a right to seek collection for the amounts that accrued under your judgment. I'm not going to detail this anymore because it doesn't sound like you recieved cash aid anyway.)
if the state is not involved, you can absolutely forgive all the arrears.
Secondly, I disagree with the broad post quoted above. Simply said, the state of the law with regard to waiver of support arrears is unsettled at this time. In fact, prevailing case law would suggest that such arrears actually owed (as opposed to those subject to a dispute), cannot be waived. This is mostly from a policy perspective that support is owed for the benefit of the child.
As a practical matter, if you were to waive them and close your DCSS case (assuming no current support), and, you never pursued them again, they are waived.
If there is no current support owed, and, for some reason, you do wish to release the DCSS pressure on your ex, you may simply close your DCSS case, without waiving the arrears, thus, enabling you to seek enforcement in the future, if you so desired. (But, if you do this, you should make it clear to your ex that you are not waiving them.)
Good luck to you.