In the Lapis Ward talk to the woman in the building right of the Pokemart she will say his son vanished
Go to the Abandoned Power Plant when the weather is windy and the boy will be there trapped by a Drifloon that you have to defeat or capture, he will say he will go home.
Then travel to Jasper Ward to the southeast of the Pokemon Center; the location of one of the missing officers. The boy is trapped by two Level 29 Venipedes and a Level 36 Scolipede. After you defeat them he'll go home
Then go back to the woman for her to reward you with a Department Store Sticker