I think it was mentioned? Jang or Jae something? My best guess is that he's Korean and J.J. was his English name. If Jang is his last name then his first name could also be starting from letter J, hence the English name J.J. But yeah, I think no one really mentioned J.J.'s real name since everyone around him isn't Korean.
WAIT I JUST RETRACED, THEY MENTIONED HIS NAME, It's Jaejin, that means the JJ literally came from his first name!
WHERE DID U EVEN FIND IT? i guess i didn't paid enough attention to the story. well, THANK YOU SO MUCH FOR SHARING THE INFO^^
i finished the story without knowing j.j.'s real name (was it even mentioned? )