You may institute discovery (questions in writing and requests for documentation) to seek proof of what is being shown to the court. If you do not want to learn the process by studying up on the rules of court and the procedures to follow (which you certainly have a right to do) then you should consider retaining a lawyer to assist.
It is his burden to disprove the amount that was set by the court and prove why he can't make that amount. The child support agency can get his tax returns and bank records to help determine his true income.
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