If you have a Comcast gateway, you can audit and monitor your device usage by going to Xfinity xFi. (Click here)
Thank you for your response.Yes but couldn’t they sell it through their smart phone? And I wonder why my usage is high this month but it keeps going from device to device that is using it the most.
Please go to the link I provided - you can then audit and even block any devices you find suspicious.
Of course, if you don't control their cell service, then there's nothing you can do about that, but at least your WiFi network won't be part of any such scheme.
And BTW, if you really think your son is up to something illegal, then you may want to contact law enforcement about it.