Talk to the man that brings you to Iron Island, he'll say that his son is ill, go to the his house and talk to the kind and the mother, than go back to the man and he'll bring you to the insland where you can find Cresselia, when she flyes away you'll find the feather to cure the kid.
Chat with our AI personalities
in the I.N.N in the canalave city (you must have member card and the boy in the canalave city sick and you awaking the boy with lunar wing)
keep trying
You can't get it without first getting the National Pokedex. When you get the National Pokedex, go to Canalave City and go to the house next to the sailor. Then he'll be in bed. First talk to him, then his mother.
the sick boy in Pokemon diamond is in canalave city in the house on the very bottom left corner .x
In Canalave city up ahead the Pokemon center is a harbor inn. There should be a house in there and in it will be the sailor's son. Talk a little time with him them go to sailor that took you to iron island, and he will be able to take you to new moon island and there you will find the legendary Pokemon cressalia as well as an item to cure the sick sailors son.