User blog comment:Dexav/Natsu's Age/@comment-96.236.191.12-20131007032250

Or Zeref could have known about him after Atlas came back to the Dragon world 400 years ago right after his first confrontation with Natsu trying to eat him. He probably told Igneel about his future son, and then somewhere along the lines Zeref probably found out about him too. Also to answer that whole "how come the Dragon Slayers couldn't get out?" question, there was probably some hidden code in there because they porbably knew that Gajeel and Natsu were super powerful.