Child support knocked down to $15 a month for my age 17 sons and he has not paid the arrears from 2 years ago.?

We have lived on public aid for two years. My husband took off with 3/4 million$ and flys to St. Thomas to work. State worker says child support cannot be modified at age 17 because the state does not have the time to return the amount before age 18. Worker is not my lawyer and cannot id the law.