Like previous people have said, most buy the game for the Multiplayer experience. I have played the game for a total of 30 hours since I got it, and have not played single player campaign once. Mainly Zombies and Multiplayer.
If he wants to get Live to play online with other people, you need a ethernet cable from your router to your xbox, or a wireless connector for the xbox which you can buy (im not sure if newer xboxes have wireless built in?)
Once the xbox is connected, he needs to buy a Live pack; and as previously mentioned ranges from 1 month, to one year. The more time you buy at once, the cheaper it turns out to be. You can pick up a live pack from any game shop.
Live lets him play MANY games on the internet against, or with many people all over the world. it significantly extends gameplay on many games. With live, he can also chat to friends using his microphone free of charge.