He really likes Fallout 3 and Fallout New Vegas, but I've never played. Could anybody give me a run down on what the game is like?

I recommend looking at the History and Lore videos in the following list, although there are a couple of other ones worth looking at

https://www.youtube.com/results?search_query=FAllout+a+history

One thing that would fairly easy to make is Nuka Cola, there are several websites with instructions on how to make that. And, you can even find some of the Nuka Cola bottles on eBay (they were given out as a game promotion to Media during the Fallout 3 release)

There are also people who have made Pipboys (the main interface in the game) and sites to show you how to make those.

And, if your feeling really lazy, Bethesda sells Fallout swag at the store

http://store.gamesas.com/brands/fallout.html

But, I like the idea of making him something, those are my favorite types of presents from my GF.

CCNA gave some good samples.

You could get some used bottle caps and on one of them paint a star on it. Open up a store and then do some bartering and he has to use the caps as currency. The goods could be small things as well as his present. Star bottle caps would be used for a special quest later in the evening. Hide the Star bottle cap someplace in the house and have him go on hunt for it aka a quest.

Google some Fallout concept art, or fan art, to get an idea of what the art design looks like.
Watching some YouTube videos will also help you see how the game looks and feels.