Since a son delivers his sire form the hell named Put, a son goes by the appellation of putra, he protecting his (departed) ancestors in every way.
पुन्नाम्नो नरकाद् यस्मात् पितरं त्रायते सुतः। तस्मात् पुत्र इति प्रोक्तः पितृन् यः पाति सर्वतः॥