How do you get the kid in Canalave city to wake up from his nightmares in Pokemon Diamond?

Well, you talk to the kid, then talk to his mom, then go to the sailor outside who says his son his having a really huge nightmare (not exactly but you get the point), then he'll take you to this place called Fullmoon Island where the pokémon Creselia lives. When you get to Creselia, she flies away and leaves the lunar wing behind. Take the lunar wing to the kid and he'll wake up (and if you want to get Creselia you'll have to look on the marking map like you did for Mesprit).