given that you have his declaration, contact the court.  Hopefully they s/b able to link to his petition.  Possibly the petition has a reference to where he was naturalized.  WWI military nats wee simplied; so not sure thee would be certificate of arrival that will give you actual info on the date of arrival and his name.  Check 1925 census for Brooklyn, assuming he was back in Brooklyn, to see what is recorded for the number of years in the US.  You can see if it matches w/ his dec.

Johanna Becker
Newport, RI