Sounds like the joint sponsor is now self employed. The current income you will state for him is the in 2023,number labeled "Total Income" from his latest tax return, 2024, unless his change to self employment happened during 2025. If he became self employed, then he has no income related to his work. He only has business revenue. Details matter in this. Perhaps he has other income or sufficient liquid assets to qualify, but there is not enough information about his exact circumstances to properly advise as to how to complete the affidavit and/or what additional documentation to apply.
When and how did this status change? Was he always self employed when he owned the business. Often the answer to that question would be no, but from what you say, he is self employed as a contract worker now.