One remark:
It's not tamper proof (you change the clock and you have internet access again or you run those commands yourself - given you know what is "wrong" with the internet connection)-zzynx
I don't know what OS you're running or whether you're using separate accounts but if you set it up so that your son is using a Standard User account, (you'd be an Administrator naturally), you might be able to set policies so that a normal User cannot change the system time, alter network settings or change Scheduled Tasks.
SJ or 40 can better answer that question.
There's not much you can do that won't be tamper-proof - anything you can do as a user can be undone by a user with the same privileges usually.

I see 40 has replied, he might be able to answer the following: Standard User as defined above can't change System settings that affect other users - does that include time, network and tasks?
Another:
Would it be possible to alter that task remotely?
Easily, just set the machine up for Remote Desktop, (or one of the many flavours of VNC, etc), and you can log in and edit the task.