The US population back then was 179.3 million. It is now 342.2 million- third most populous country on earth.
He also failed to factor in rise in the cost of living.
Nor does he and others comprehend the compounding flex design of Social Security that adjusts for both the workforce and beneficiaries. Among these is more concern for the border, not seeing immigration, legal or not, assimilated as quickly as possible into the US workforce that PAYS the Social Security he draws. What he paid into it was drawn for retirement by HIS parents.
Don't try to explain it. Explanation only makes him mad until he apologises to himself on your behalf.