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If you want to just stop him installing software (i have not read through all the posts) then here's a solution that will test his sys admin skills a little.
This can, of course be extended (update-manager, software etc). You can script it on your login to enable it and disable on logout of your account.Code:matthew-S206:/home/matthew % which apt-get /usr/bin/apt-get matthew-S206:/home/matthew % sudo chmod -x /usr/bin/apt-get matthew-S206:/home/matthew % sudo apt-get update sudo: apt-get: command not found matthew-S206:/home/matthew % sudo chmod +x /usr/bin/apt-get matthew-S206:/home/matthew % sudo apt-get update Ign http://security.ubuntu.com quantal-security InRelease Ign http://extras.ubuntu.com quantal InRelease Ign http://gb.archive.ubuntu.com quantal InRelease <snip>
This is one of a number of solutions you may consider.
If you believe everything you read, you better not read. ~ Japanese Proverb
If you don't read the newspaper, you're uninformed. If you read the newspaper, you're mis-informed. - Mark Twain
If he has physical access to the box and is capable of starting in recovery and changing passwords then you can do nothing at a software level. I suggest you discuss the use and missuse of geek skills. Id get him his own box with no OS and let him download, install and play.
IT skill are a great thing to develop, problem solving skills even better.
People keep saying "physical access is root access," which is true, of course, but then seem to also imply the child must have physical access in order to use the computer, which is absolutely untrue.
This may seem a bit over the top, but you can set up a kiosk-type situation, in which you have a Linux desktop computer locked in a box, with enough of a hole to let up wires for a keyboard, mouse, and monitor. Short of your son taking out a saw and sawing open the box (or successfully picking the lock), he essentially no longer has physical access, and thus cannot remove the CMOS battery or boot up a live CD/USB or bypass the Grub password.
Another way to go about this is similar but a bit different. Supposing you don't have or don't want to purchase a kiosk-like box. Lock the computer up in a room or closet and enable openssh-server or VNC so your son can remote in from another room and play around with his login... remotely. This is similar to how most companies are set up with Linux servers (can apply to desktops, too, in this case). You've basically set up a "server room" for a desktop.
P.S. I know it's very frustrating to be getting parenting advice from people when you're looking for a technical solution, but it's also equally frustrating for people here you're asking help from to get the condescending "I was using Linux before you were born" bit.