Because the son rescues his ancestors from the hell, called Put, therefore he has been called by the self-created (Brahma) himself as Putra.
पुत्राम्नो नरकाद् यस्मात् पितरं त्रायते सुतः। तस्मात् पुत्र इति प्रोक्तः स्वयमेव स्वयम्भुवा॥