Reddit mentions of microtivity IB400 400-point Experiment Breadboard

Well, breadboards run pretty cheap... And size matters here. Sometimes a smaller board is better, but a large board gives you more room to work with. So I'd recommend getting both! A small board will be nice for smaller projects, but larger boards will be nice for more in-depth prototyping that require more room to work with. And he can always use a small area of a large board when he doesn't need it all... but a large board won't fit into a small boat or airplane if that's what he is doing... So, at a good price, both can be beneficial.

Other than that, he'll need jumpers (male/female), and maybe a ribbon cable. Ribbon cables need a T-board (or equivalent) to connect to the breadboard with, but jumpers just plug into the GPIO pins. With a small breadboard, jumpers are favorable, as they only take up as much space as one pinhole, and they come in many different colors and lengths. A larger board can accommodate a ribbon cable and T-board more readily... a T-board can take up a LOT of space on a small breadboard REALLY quickly.

Word to the wise: if you buy a ribbon cable and T-board, you are going to want to know what type of Pi he has. The original versions had fewer GPIO pins than the B+ and the A+ versions. This means that a larger ribbon cable wouldn't work with the original Pi's. If he has an A or B, a 40 pin cable/T-board wouldn't work as they're designed for the newer B+ and A+. If you need help identifying which board he has, we can help you with a photo or a description of the board...

Also, a resistor kit and a capacitor kit would be good to have, if he doesn't already have them. Most projects require that you use resistors/capacitors somewhere, and the kits usually come with several of the most used resistors/capacitors, which should tie him over for some time.

Then you should get him an LED set. LEDs are nice because they can be used as a "proof of concept" tool. Since everything into and out of the Pi is digital, all you are doing is giving an item voltage. So it doesn't matter if you're using a buzzer, motor, servo, or LED... they will all work interchangeably. He can use an LED to prove that his code and wiring works when the light comes on... then, when it all works, he can swap the LED for a buzzer or something, and everything should still work, just with sound instead of light.

You could also buy him some sort of case, but these aren't always necessary. I don't use on on my Pi, and it works just fine... I just have to make sure I'm not shorting it on anything, or breaking it in some stupid way. These vary so much in design and price, I'll let you search for them instead of providing a link... just remember, like the ribbon cable, cases will obviously depend on the model he has.

Personally, I would stay away from potentiometers and photoresistors since the Pi only has digital inputs. Both of these items would need to be attached as analog items to work properly, and that means buying extra boards for correct functionality. The Raspberry Pi can not natively read analog inputs, so these items would only be frustrating to him since he can't use them without extra parts.

This should get him started in coding and prototyping. If he's looking for motors, servos, or speakers, you can get him those too, but they're not necessary to tinker with a Pi.

There are also kits that offer these items, but they're usually more expensive than they have to be, and they may or may not have everything you're looking for. Also, there are other places to shop for these types of things, but I use Amazon because it was the quickest and easiest place to search for examples of all of these things for this post. If you're smart with your shopping, you can get all of these items for ~$100, and might be able to have them shipped before Christmas... but you're going to have to hurry.

EDIT: Added links since you said you knew "nothing" about these items. This should make searching easier since you can actually see what I'm talking about...

http://www.amazon.com/microtivity-IB400-400-point-Experiment-Breadboard/dp/B0084A7PI8/ref=sr_1_2?ie=UTF8&qid=1409954029&sr=8-2&keywords=breadboard

Any breadboard will do, just be sure they are not the small sized ones, otherwise you will run out of space soon. You also need a Pi Cobbler to connect the pi to the breadboard. There are some really good starter kits out there that only include breadboards, some wires a cobbler, some leds and a switch, have a look on Adafruit as well.