How can I find my son’s iPhone using my phone

If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

He didn't have to set up the app he just needed to set up iCloud and turn on Find My iPhone along with having location services turned on. Use the link I provided to find out how you can log into iCloud or use your app if he indeed has it activated.

Did he set up the Find My app on his device?