This is a very good question. $80 per month is less than what he'd be ordered to pay if making minimum wage. Think about using a private lawyer
I am not intending this to be legal advice, because I don't know the particulars of your situation. Call me if you would like to discuss this or other isues.
If by "working under the table" you mean that he is hiding income from the court, it is also likely that he committed perjury in a court proceeding. This is a felony in most states and could get him held in contempt by the judge who issued the support order. The real question is whether or not you can PROVE that he has unreported income. Contact an attorney who can schedule a hearing on the matter and, in preparation for that hearing, take the deposition (a sworn statement) of you son's father's employer and require him to bring all records pertaining to that employment (pay records, check stubs and canceled checks, timesheet, work orders, invoices to customers that list his name, etc.).
Only an attorney can get the information you need. Under certain circumstances, it is possible that your son's father can be ordered to pay these attorney's fee.
The problem here is that he has figured out the AG system -- if he pays something every month they stay off his back. You will need to hire a private attorney who will file contempt of court. A judge who hears the case will give him an opportunity to catch up and when he doesn't, will throw him in jail. That will get his attention. The AG has over 1 million cases in their system. Don't blame them. Attorney's fees are mandatory in these cases so you may be able to find someone who will take this case for a little up front -- especially if you have good information on the obligor. Look here on Avvo for lawyers in your county who do child support. There are a lot of good ones. Good luck.
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