AOS 864 joint sponsor RFIE for evidence of current income? How?

[whhv]

My spouse and I currently are currently unemployed, so I asked my relative for a joint sponsor. The joint sponsor owns a small company, has no employees, and works as a housing agent. But the majority of his income comes from the rent he collected from his tenants, so he doesn't have any W-2, 1099s, or pay stubs, he got all money by cash. His most recent three-year income is always around 40k, half of it from the rent. However, after I submitted my joint sponsor's i-864 and three-year tax return (IRS transcript), I got an RFIE from USCIS about the Affidavit of Support (i-864), they specifically asked for evidence of current income, like pay stubs or other documentation of pay for the previous 6 months, etc.
Are there any other documents that could prove current income? Any ideas would be really appreciated!


Here is the RFIE i received from USCIS: