Assuming it's a Windows PC you can do it with a simple script:
1) Get hold of shutdown.exe from the Resource Kit
2) Write a batch file or script that calls it after an hour and shuts the PC down
3) Stick the file into his startup folder so it runs when he logs on, or if he's too clever for that, call it from the Run key in the registry
Shutdown.exe is great. You can shut down and/or reboot both local and remote machines, and you can even make him lose any unsaved work if you're feeling really mean. For the sake of family harmony however I recommend you script it something along the lines of:
(50 minutes) Time is nearly up
(55 Minutes) System will shut down in 5 minutes, save your work
(56, 57, 58 Minutes) Like to push your luck don't you?
(59 Minutes) Don't say I didn't warn you..
(60 Minutes) Shutdown!!
