Go here - http://www.germangenealogygroup.com/records-search/naturaliz... Look under Database and then Naturalization. See if you can locate the correct record and sent for it. The first page of the Database provides information on now to order. Also, at the top of the page is a link to Kings County Database (just in case he applied in Brooklyn)

The complete document will have more info - hopefully they included the city. The city they came from may have been in Poland when they left and in Russia by the time they came to the US.

Good luck. Happy New Year