If Social Security is his only income, he will not need to file his own return.
I am the guardian of our adult son age 24. He gets social security on my account and pays my wife and me rent. He lives in the same house with us. Do we have to declare rent income on our tax return?
It depends on whether he pays you rent that is above, at, or below fair market value. If he pays you rent that is below fair market value, then no, you do not need to claim any rental income on your return. You can still potentially claim your mortgage interest and property taxes on Schedule A, but you wouldn't be able to take any other "rental" expenses. If he pays you at or above fair market value rent, then yes, you do need to report the rental income, but that also opens up some additional expenses to be taken on your return. If you're not sure what fair market value would be, look for similar properties (for example, one bedroom with a shared kitchen, if that fits your situation) available for rent in your area, to start.