Is he still liable though for the back support and any interest that continues to accrue on, as it was not forgiven in the court records?

My two daughters were adopted by their stepfather in 2010, after having no contact at all from their biological father since May 2004, so it was granted on abandonment and such. Everything was posted in the paper and his current PO Box in Oklahoma is where copies of all these court documents went. He has $33,000 that he was behind prior to that in child support. I did not have that waived the court document just states that no more will accrue from this date of the adoption August 2010 and after. He never called, wrote or showed to the hearing. This is an interstate issue since I reside in Nebraska and he resides in Oklahoma.