4 Answers

Pay the money for him. They do not care that he is not working. People lose their homes because they miss their tax payment on the property for just one year because they can't find a job. The state and government always gets what's due them or it's off to the clinker.

Pay the money or let him go back to jail. Sadly he didn't find work so someone will have to pay... If you don't have any money you may have to sell something the things people have to do for their children.. Good luck

Ask if you can pay part of it, or what you can afford each month, don't ignore it , they won't go away.

People are right; no one in authority cares where the money comes from, just so they get it. My kid owes thousands and no one is getting anything. 
If payment is a condition of your son's probation, he surely could be locked up for not making his payment.
I'd suggest he set up a payment plan with the Revenue Collection Unit to make some stipend of payment on a monthly basis.  Even $5.00.  Surely you or a friend can find something for your son to do to earn $5.  Instead of giving it to him, send it to the collection unit. 
In Fresno County, our RCU will have the state's Franchise Tax Board confiscate a state income tax return to pay off a debt.  The IRS issues orders to withhold paychecks for DMV registrations, among other things.   Getcha coming and going.