User blog comment:FairyRaven/Fairy Tail Chapter 435 & 436: The Cry of Victory & Memoirs, Review and Commentary/@comment-24277053-20150518124050/@comment-9595671-20150518152742

I'm not so sure, if he did restore Natsu then it wouldn't have taken so long.

Unlike Zeref, Natsu isn't immortal because Zeref became immortal for trying to revive Natsu. I'm pretty sure he gave his brother a new "soul" which is why Natsu doesn't know they're related.