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Find Friends app is more suitable and faster than Find My iPhone, because it saves you the entering of your sons password all the time you want to see where he is.
Add him with his mobile number to find friends. He will get a notification that you requested to follow him on Find Friends. After / if he accepts you'll be able to see where he is until he decides to stop sharing his location with you.
For "Find My iPhone" to locate your son's device, all you have to do is initiate the app on your device and then enter the Apple ID and passcode associate with your son's device/account. By the way, I'm assuming your son's device is powered on and has the Find My iPhone option enabled.