If the Social Security benefits are your son's only income, he does not have to file a tax return unless he received advance premium tax credit for health insurance from the Marketplace ("Obamacare"), or he took money out of a Health Savings Account (HSA) or Medical Savings Account (MSA). If he did receive advance premium tax credit he should have gotten a Form 1095-A. If he took money out of an HSA or MSA he should have gotten a Form 1099-SA.
You do not report the household expenses that he pays for on your tax return, whether or not you claim him as a dependent.
