Dear Ms Bottenberg,
Thank you for posting your request on History Hub!
In most cases, the National Archives will not have a copy of his certificate of citizenship. Two copies of the certificate were created – one given to the petitioner as proof of citizenship, and one forwarded to the Immigration and Naturalization Service (INS). All INS records are now overseen by the US Citizenship and Immigration Services (USCIS). We suggest that you submit a Name Index Search to the USCIS to obtain a copy of your grandfather’s files in their custody. When you submit an Index Search Request, the USCIS Genealogy Program will send you a letter reporting the search results. The letter will identify all file references found in the index and provide instructions and additional information necessary to request the naturalization file(s) from the USCIS Genealogy Program.
Since you are pursuing dual citizenship, the Consulates are aware that USCIS does NOT provide certification of files in their holdings. Instead, you must present the photocopy of the document(s) along with the USCIS Genealogy Program response letter and mailing envelope.
NARA does not have the authority to issue an apostille. We suggest that you contact the Office of Authentications - U.S. Department of State to submit a request for Authentication Services.
In addition, NARA’s website about Dual Citizenship Assistance - Frequently Asked Questions may be useful.
We hope this is helpful. Best of luck with your family research!