Seeedstudio.com (yes..that is three "e"s) has a 20 percent off sale but you have to guy their gift certificates before Christmas. The store ships from Hong Kong so I don't know if they would get it here by Christmas.
The first thing he needs to learn is how to blink an LED (light emitting diode) to get an understanding on how the Arduino works.
You probably want to get him a starter kit with an LCD, a couple of breadboards, a servo, a piezo speaker to make noise and some LEDs.
There are places that you can buy packs of resistors and capacitors really cheap instead of paying .25 or .35 cents for an LED.