Sorry, I misread your post, thinking your "boyfriend" was also the father of your children and he owed the money to you.
It's not a crime to owe money. In some states it is quasi-criminal offense to fail to pay child support. Since his kids are grown, he would not seem to have an obligation to currently support them.
Call a lawyer. Chances are the State will take whatever money they can get (which might be pennies on the dollar) and then drop all claims against him.