He is in Canalave City and you need to have the National Dex first.
Chatot, you can catch him on the way between Valor Lake front and Sunshore City. Or in Enterna city trade a buisel with a boy in the Re-Nicknamer's apartment
keep trying
the sick boy in Pokemon diamond is in canalave city in the house on the very bottom left corner .x
He doesn't get sick, he has a nightmare about darkrai darkrai just made him have a bad dream
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)
the sick boy stays in the sailor's house in canalave city after you get the national pokedex.
there is a house with a sign by the dock. In platinum, you have to beat Cynthia for him to be sick.otherwise, he's already sick.
to find which boy you have to heal, find the house closest to the ship in canalave city, next enter the house and you will see a boy in bed.
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.
be mean and tell them they will have no chance with you and to stop with their sick fantasies of you.
Sick is normally an adjective She is sick. That ride was sick! :) The sick boy stayed home.
After that you go to Canalave city and give the lunar wing to the sick boy. After that it is randomly in the wild.