Where's my nii-chan?

This is my first story so please don't judge. Also I'm not good at explaining things so I may pictures to show characters settings. Also feel free to give me tips to improve my writing in the comments.

I don't own Fairy Tail. All characters belong to Hiro Mashima.

After Team Natsu finished their mission, a mysterious person shows up. He doesn't say a word when asked questions. He just teleports behind them and shoots a spell at Natsu making him pass out. In a hurry to get him help, the rest of the team rushes to the guild and ask Wendy to heal him. After a few hours he turns into a little kid. And to make matters worse, the black wizard Zeref suddenly appears by the guild doors.

Why is Natsu a kid?
Does he remember anything?
How do they turn him back?
Why is Zeref here?