You have to talk to the man that takes you to iron island and make him go to fullmoon island. Go upwards and you will find a Cresselia dropping feather i don't remember the name of. Then you will have to talk to the boy again and he will be cured.
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the sick boy in Pokemon diamond is in canalave city in the house on the very bottom left corner .x
i think it happens when you beat the elite four
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)
As you advance in the game, you will find that you are able to go to Iron Island from the cruise ship in Canalave City. When the game is beaten, the ship in Snowpoint City Will be able to take you to the Fight, Survival, and Resort Zones. The Battle Tower is there. The final place that will become available is Half-Moon Island, which is again in Canalave City, after you cure the sailor's sick son.
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.