How to apply for deceased father's Certificate of Naturalization?

Alien son of a naturalized U.S. citizen is applying for his U.S. citizenship. US Embassy was asked for a certificate of naturalization of his father who is dead.
He does not have a copy of his dead fathers certificate of Naturalization. Its lost. He has dead's fathers US passports and other US documents. How can the alien son obtain a certificate of US naturalization of his deceased father?