Instructions
Talk to the young boy's father, which triggers the event in the game where he asks you to go to a place called "Fullmoon Island." Accept his request for help and follow the sailor's father to this location. "Fullmoon Island" will be located on your in-game map in case you need help along the route.
Explore "Fullmoon Island" until you find the character Cresselia. Speak with Cresselia and continue to talk until the character leaves. The character will leave behind a feather, which can be used to wake the boy in Canalave City up from his nightmare.
Return to Canalave City with the feather you received on "Fullmoon Island." Speak with the boy, using the feather to wake him up. His father will be thankful.