If he is failing to pay under court ordered child support, you are entitled to back child support and current child support. If he didn't have a job, he should have filed a motion to modify the support. Because he failed to do so, the current support order is in play regardless of his income.
You should hire an attorney and file a motion for contempt. In the motion, you can request back support. Although his defense may be that he is unable to pay, the court is not likely to decrease the back support. However, from the point of the response forward, his monthly support may be decreased downward.
You should hire a local attorney to start the court action. If you cannot afford an attorney, it is likely that your county or the surrounding counties provide pro bono programs for low income family law parties.