The Court may and often does imput income to someone who is voluntarily underemployed. Key items to focus on would be previous years tax returns before he started "reducing" his income; and, bank statements to attempt to catch some of the cash deposits from "under the table" jobs. You will need an attorney to help you properly request or subpoena these records.
You face the difficulty of someone being self employed and many times we see these issue of when someone quits there job just before the next child support hearing. The rules are the same. You can not intentionally hide or reduce your income and hope to get a reduction.
The good news is that you will not need to. IF you Ex requests a modification based on a lack of employment, the court will have toy determine that he is unemployed through no fault of his own, and that he has been diligent in finding work. He will have to detail where he has applied and what efforts he has done to find work. If the court believes he is not voluntarily unemployed and working diligently to find work, only then can it consider amending the support obligation. I have even seen judges go further and ask the person how exactly they are living without any income, and what bills they have. If unsatisfied with the answers, the judges do have the ability to impute income based on what they made when the order was entered, in order to inspire the person to work harder to get his employment situation cleared up as quickly as possible. The advantage you have going in is that few judges want to amend child support. Convincing them to do so is quite a difficult task.
Whether his support will be reduced or not depends on the evidence that is presented to the court. If he was terminated from his previous employment then his former income could be imputed to him, which means the court would hold him to that amount of income. If he was laid off or lost his job through no fault of his own then the court may not impute income. That being said, that doesn't mean that he does not have to take meaningful and significant steps to try and obtain employment. If you can show that he is not making a good effort to find new employment, even if that employment is outside his general area of employment, then the court could hold him to certain income.
If your former husband presents evidence that he is actively searching for jobs and has been, through no fault of his own, unable to find a job then the court could hold him to a lower income than he previously made. That income could be minimum wage.
If you believe your husband is voluntarily underemployed you will need to hire an attorney. An attorney can take certain actions such as issuing subpoenas to former employers or current part-time employers for records that will bolster your position.