Generally you'll get 3 times more value out of each buck you spend if you choose a desktop PC for gaming, instead of a notebook. Unless your son likes to play World of Warcraft at school (assuming it has internet access and its allowed) a desktop PC would be much more logical. World of Warcraft cannot be played without internet access.
A new desktop PC of about $400 (excluding monitor) would be excellent for playing WoW, with a monitor that would be max $600. You'd need about a $1500 laptop to match the same performance level.
One piece of advise: spend some money on a good mouse, preferably one with a cable and laser optics. This is important for high-pace games like world of warcraft. I use a Logitech G5 laser mouse and its just perfect for this type of game.