Nepalese Passport and English translation of Nepalese citizenship do not work as a birth certificate replacement for USCIS. You have to get a statement from your VDC/municipality office (in their official letterpad) from Nepal that his birth record doesn't exist, then you should submit that letter statement along with the secondary evidences of birth like copy of passport, copy of Nepalese certificate with english translation, birth affidavits like that.