During our divorce in 1992, my ex voluntarily relinquished his parental rights. He was not ordered to pay child support. Could he now be ordered to pay child support?

Yes. He may or may not retain any parental rights (see "sticky" on that subject near the top of this Family Law sub-forum). But that doesn't mean that he is not still obligated to pay child support. Was the child adopted after the RPR was signed?