If you press the del key during the system boot up yo can enter what is called the BIOS (Basic Input/Output System (I think it means)). It gives you and option for setting up all your hardware at the systems basic level. Its not really for beginners to experiment with, BUT there is an option to enable what is called a supervisor passed and a user password. It can prevent people from entering the bios setup as some changes can harm your configuration. You can also set this so that your system asks for a password when the machine first initializes. I strongly think this it what has happened.
Your son has set up a password in your bios and set it so that you have to enter this before it will boot up.
Possible soloutions are... Take a big long peice of 2" by 4" wood and hit your son repeatedly until he confesses and gives you the password!!
Or another way is...
There is a small "Jumper" located on your motherboard, so get out ya screwdriver and open ya beast of a machine up...
The jumper is located on the mainboard near a lithium battery cell, you know the button type.
If you can determine what motherboard you have then the diagram inside the manual will tell you where the BIOS reset or clear jumper is.
What ya need yo do is...
Make sure your machine has no power first!! Switched off etc..
The jumper is usually a black or white plastic coated square thing, really small about 6mm x 2mm x 8mm (I didnt measure it btw jus a guess) and it usually sits on the motherboard where 2 pins are sticking out and it sits on one of them.. What ya have to do to remove the jumper and put it on both pins for about 5 seconds and then put it back into its original position, or things will not work ... Sometimes, there will be 3 pins on the board, its like a crude type of switch so just move it over, 5 seconds, return to original position...
Now what this does is RESET your bios configuration, it will remove any settings including the password which has been set. It could be a real pain trying to configure it again.
There are plently of sites around to help you configure your bios so not to worry, although it can be tricky if your not used to doing it as some things are a little technical for a beginner
Hope this helps and if your stuck for some 2" by 4", use a baseball bat
Just kidding.. !!!