Do you have a wireless internet connection in your house? If so, you could just change the password
Here is the information regarding
Wii internet connectionMost of the electronics that kids have nowadays are possible to connect. Mobile phones are another one to watch as he will be able to connect whenever and wherever he likes.
Without a wired or wireless internet option in your house he'll still be able to connect using a
mobile connection 'dongle'.
You either have to be able to trust him, or remove his 'toys' for a while. Good luck with that :)