This link might be helpful as it explains the WiFi connection,
Make sure that the WiFi in the settings menu is always turned on and connect to the WiFi at your house. Also use the DataSense App that comes with the phone. Set up a limit that warns your son he is at 50% of his data limit. It will also restrict data for apps when the limit is close. A truly wonderful Windows Phone feature. On top of all of this the more WiFi locations he connects to the more the phone will map out WiFi and connect as much as possible to it.