Do you mean Social Security on a SSA1099----or does he get SSI? Those are not the same. If he gets SSI there is not a tax document to enter.
Go to Federal> Wages & Income>>Retirement Plans and Social Security (SSA1099 and 1099RRB) to enter your SSA1099.
WHAT IS THE DIFFERENCE BETWEEN SOCIAL SECURITY BENEFITS AND SSI?
https://www.ssa.gov/ssi/text-over-ussi.htm
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