Question about income of self-employed

My fiance owns his own business which is incorporated. He has child support he has to pay and the court wants to garnish his wages because he has gotten behing. We are unsure how to provide proof of income, When he told the child support women that he was self employed she said that he needed to get a real job so he could prove how much he makes and they could garnish his wages. Anyone else ever had this problem if so please let me know he has a court date in the next month.