The son is called Putra because he frees his ancestor's debt. Through sons and grandsons one always enjoys the happiness of those regions which are reserved for the pious and the good.
पितृन्णात् तारयति पुत्र इत्यनुशुश्रुमा पुत्रपौत्रेश्च नियतं साधुलोकानुपाश्नुते॥