User blog comment:PUNCHURPREGNANT/Gray and Silver Predictions/@comment-41.220.47.249-20140314151419/@comment-25244540-20140323021826

Zeref did seem to know Natsu before Natsu met him, but the question of how he knows Natsu is still unclear.

I don't think they're related unless Natsu's his descendant as Zeref has been alive for years and we;ve seen Natsu when he was young so he had aged over time.