Do you own the property that your son is paying you rent for, to live in? Does your son live in a property that is not occupied by the owner of the property as that owner's primary residence? If so, then it's rental income that gets reported on SCH E as a part of your personal 1040 tax return.
It comes from his SSI check each month.
Where it comes from doesn't matter. Rental income is rental income and it's reportable as such.
Must we amend our 2018 and 2017 returns then?
If you received rental income in those years and did not report it, then yes. Since you are required by law to depreciate rental property, it is imperative that you amend your returns in chronological order starting with the oldest tax year you first rented out the property. This is the only way to ensure the correct carry over of depreciation and other losses that will probably occur.