The Medicaid expansion state exemption applies to all income levels below 138% FPL, including below 100% FPL. You enter exemption code 'G' in Part III of IRS Form 8965.Now that we have discovered that he lives in TX, everything changed, though. His quickest route to an exemption is to file his taxes for the Medicaid Expansion state exemption. The income he stated is between 100% and 138% of FPL. So, whether it's Social Security benefit or taxable private insurance, he should be able to secure that exemption.
If it's non-taxable private insurance he can't get the Medicaid Expansion state exemption, because his MAGI is below 100% (Medicaid eligible). So then he must use the Affordability exemption.
In non-expansion states, being below 100% FPL does not automatically create Medicaid eligibility.