If you were divorced in VA, you can file a rule to show cause against him in VA to enforce the agreement and hold him in contempt of court. If he is working "off the books" he might be able to request a modification of the support amount based on his current income, but he still owes you the arrears for the amount agreed to. Good luck.
If the agreement was adopted and incuded as part of the Final Decree of divorce it has the force of a Court oder. File a Petition for Criminal Contempt. If it was not made a Court Order, you could sue him on a contract theory. In my experience, if you let him slide or tolerate non-compliance, he will get worse, not better. Take action now.
If your Final Decree of divorce incorporates your settlement agreement, then your ex-husband is in violation of a court order. If you were divorced in Virginia, your Final Decree of divorce from the Circuit Court should state that issues related to child support, visitation, and custody of the child are remanded to the Juvenile and Domestic Relations court in the jurisdiction where the Final Decree was entered. If you and your son live in Virginia, it sounds like that is the proper jurisdiction to file your motion. A Motion to Show Cause for Contempt of Court can be filed for an alleged violation of a court order concerning child custody, visitation or support, and the contempt may either be charged as civil or criminal. You should discuss with an attorney whether civil or criminal contempt is the better option and discuss the advantages of filing in Circuit Court or Juvenile and Domestic Relations Court to enforce your order. The JDR court usually has jurisdiction to modify an order regarding child custody, visitation and support, but both the Circuit Court and JDR court have jurisdiction to enforce the court order.
If the divorce was granted in Virginia, then you should be able to proceed against your ex husband in Virginia. If your agreement was incorporated into the final divorce order, but not merged, you can proceed against your ex through contempt or breach of contract. If you attempt to hold your ex in criminal contempt, he will almost certainly be able to defend using the higher "beyond a reasonable doubt" standard. Accordingly, you might want to consider attempting to hold him in civil contempt. If the agreement was not incorporated into the divorce order - that is, it was entered into outside of the divorce process - you cannot pursue contempt until the agreement is incorporated by the court. In addition, once it is incorporated, you might not be able to obtain arrearages under the agreement under a contempt action (because the agreement was not an order until the time it was incoporated). For self-employed, or "off the books" workers, one good way to determine their income is through a review of their bank records - by adding up all of the deposits, an argument can be made that the total amount is the "income". Another way is to argue that the total amount of expenses shows the income, because without sufficient income, your ex couldn't afford those expenses. Good luck.
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