Looking for census or naturalization information for my grandfather who entered USA in 1911


I have found in Ellis Island records that my grandfather Costas Antonopoulos, arrived in USA on May 12th in 1911 at the age of 25. I tried to find information about him in the census records or naturalisation records but I didn't have any luck. However my father told me that he returned to Greece during WW2 but I can't find any information in the departed records from 1920 and onwards. 

I would like to ask if someone can help me to find information about his life in USA and if he returned to Greece permanently or he became a US citizen. 

Thank you in advance

Andrew Antonopoulos