when you say we are both US citizens, I assume this refers to your father and you. Also, since you say he's receiving Social Security, I'm also assuming he's over 65. One filing requirements depend on gross income, Gross income includes gains, but not losses, reported on Form 8949 or Schedule D. The sale of the land would be reported on schedule D. so only the gain, if any, not the gross proceeds would be taken into account
Gross income means all income received that isn't exempt from tax, including any income from sources outside the United States. Don’t include any social security benefits unless (a) you are married filing a separate return and you lived with your spouse at any time in 2018 or (b) one-half of your social security benefits plus your other gross income and any tax-exempt interest is more than $25,000 ($32,000 if married filing jointly)
once you figure out gross income based on above filing is required
if single and gross income at least $13,600
married filing joint $25,300 if only one is over 65 or $26,600 if both
married filing separate - just about any amount
(these amounts are slightly higher for 2019)
even if not required to file an income tax return a gift tax return is supposed to be filed.
can't say everything that the IRS would do to your dad if they catch the non-reporting if a income tax return is required. At the very least they would try to collect the tax, penalties and interest due. There is a minimal penalty of about $200 for failure to file gift tax return assuming no tax is due.
If he lives in a state with an income tax, that return would also be required.
