So even if you withdraw your case and 'forgive' the monies owed to you, the state will likely still suspend his license etc. until the money they are owed is collected.. and your x will still not have a license.
Another alternative to forgiving the arrears all together (as in most states child support is considered a right of the child) most states allow you to agree to a 'hardship on arrears'.
I am in MT and a few years ago I did this for my x, he was not $15,000 behind though. All I had to do was call my caseworker, request the paperwork, have x sign with me and return it to her. It didn't forgive the arrears, but it stopped the states collection proceedings for 12 months.
During those 12 months the state still attempted to collect current child support, but did not attempt to collect on any arrears. At the end of the 12 month period we were sent a new hardship packet to complete if we chose to. If we hadn't completed it, the hardship would have ended and the state would have begun pursuing the arrearage again.