You are suggesting that your son committed a juvenile offense, and that there was a disposition after he turned 19. He was ordered to pay restitution as a result of the damage or injury he caused and, as his parent, you were held jointly liable for your son's restitution.
Joint and several liability is not about control at the time of the offense. It's about your parental duty to pay restitution.
You have apparently filed a claim of exemption in relation to your wage garnishment. We lack any information that we would need to assess whether you have a valid claim of exemption. It is reasonable to infer that the prosecutor or victim's lawyer has responded that you are not entitled to an exemption, or that some portion of the garnishment may proceed despite any applicable exemption. Do you need to inform the court that you want a hearing to determine if you qualify for an exemption? The court will have figured that out by the fact that you filed a motion claiming an exemption.

