You will find a boy crying just outside of the Poké Mart in Kevlar Town. When you first visit, he tells you to just leave him alone, but if you revisit the town after receiving HM04 (Strength) from Kellyn in Legen Town, he'll finally decide to ask you for help.
The Dunsparce Gang, a local biker gang, has stolen something from him and run off to the Passage Cave Grott-Hole, a small cave located to the far west of Kevlar Town.
To start the Crying Boy Sidequest, talk to the boy, Maury, crying just outside of the PokéMart. Leave the PokéMart ledge and take the road to the far west of Kevlar Town.
Using a Pokemon with Strength, move the boulder out of your way and go into the cave entrance.
Note: The biker gang have also paved the cave floor, allowing a rare interior where you use the bicycle
You will face three Biker-class trainers: Biker Lou, Biker Jo, and their leader Biker Richard.
Take the cave path north and battle Biker Lou.
Once Lou is defeated you will find a section with several boulders and mushrooms. Use strength on the boulder to the far left and push it as far north as you can. Coming from the left, push the the second, middle, boulder as far right as possible. Then, push the third boulder north as far as possible. You can now walk around the boulder and continue north to battle Biker Jo.
After defeating Biker Jo, take the cave path to the east and battle Biker Richard. After defeating Biker Richard, you will get Maury's missing object: his wallet.
Take the wallet back to Maury at his house, and he will give you...nothing. It turns out that Maury is a jerk who pretended to cry so that someone would help him get his wallet back because he loves money. In fact, if you visit him again he threatens to call the police on you for invading his home.
Didn't Biker Richard say something about helping people and the environment? Oops...maybe this is one side-quest you should have skipped.