You can add your father as a dependent by amending you previous tax returns. There is a 3 year time limit (2015 must be amended by 4-15-19).

Your father must not have filed a tax return of his own, for those years. If he did, he too will have to amend, unclaiming himself.

You cannot claim him, for any year that he had more than $4000 (+/-) of taxable income (social security is generally not taxable income). For full rules, see https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Ret...

How to amend https://ttlc.intuit.com/questions/1894381-how-to-amend-change-or-correct-a-return-you-already-filed