a program that i can spy on a pc via internet?? ( to see on my screen what he's doin??)

Goto
http://www.logmein.com

1.Create an account.
2.Download LogMeIn free.
3.Install it on your son's computer.
4.Create another administrator user account there & give a password.

Now access internet from your PC. Goto logmein.com.
Log into your account. And you can see what he is doing.
You can even control that PC.

And give your son a limited user account on his machine.
Install any software that will protect the logmein software from terminating.