Genealogy / Changed last name from Paprocki to Fern sometime after naturalization [2]

Hello, My great great grandfather was either Julian or Julius Paprocki when he came over in 1880 from Poland. It appears that his son John Paprocki changed his last name to Fern but I can't seem to find anything legal showing the change. My great grandfather was 4 years old when he came to the states. I can't find a naturalization for my grandfather but I find one for my great great grandfather. Any suggestions on how to find this info?