Unfortunately, I think the only option is to boot into recovery and do a factory reset. That will delete all the apps and data he has put on the phone. Apps can be reinstalled from the Play store (It will remember what you've bought) and any contacts, calendar entries and email that are in Google will be restored. But things like game progress will be lost.
I think, to boot into recovery, you turn the device off, then hold power and volume UP. When the Samsung logo appears, release both buttons. the use volume buttons to move up and down the menu, and power button to select Factory reset. (If that doesn't work, post back. Someone here will know for sure.)
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