The single player is a little short, and compared to other first person type games isn't necessarily the most exciting.
On the other hand, the multiplayer is great. Four friends playing on one XBox, or up to 16 if you link 4 XBoxes together. This is where Halo, and now Halo 2, have excelled. My friends and I had a great time the other night with Halo 2.
Playing on XBox live doesn't seem to be living up to what people thought though as you can't choose what game you want to play. When you log on, it just assigns you to a game.
Has your son had any friends over to play multiplayer yet? If not, he should. You might not like it though...
goates