Even when the child turns 18, he is still liable for any child support arrearage. You can file a motion to show cause (contempt) or contact the Child Support Enforcement Agency and get them involved.
Legal disclaimer: The response does not form an attorney-client relationship. Further, the response is not intended to create nor does it create a duty to answer additional questions. The response should not be relied upon as legal advice. The response given is based upon the limited facts provided by the person asking the question. To the extent additional or different facts exist, the response may change. Attorney Sams is licensed to practice law only in the State of Ohio. All responses are based solely on Ohio law unless stated otherwise.
SSI is not considered income. If that is all he gets his child support order should be 0 upon modification. It is doubtful that you can touch his SSI benefits. While the arrearage won't go away, he might not have assets for you to attach. You can file for contempt based on him not paying the back order.
This answer is provided for general purposes only. If you need legal assistance you should consult with an attorney. Responding to questions DOES NOT create an attorney-client relationship.