Mkay from your list, from someone of similar age to your son (I'm 18), who's also into FPS games;
Far Cry 2 Unpolished, poor performance at times (regardless of specs), slightly off hitboxes and not much in the way of longevity. I wouldn't recommend it.
Dead Space Simply not an FPS. It's a survival horror, although it does have a lot of action, one of the main focuses is the storyline, so if that aspect isn't of any interest to your son, again I wouldn't recommend it. That's not to say the game is bad, it's very good in fact.
Tomb Raider: Underworld Just no, Tomb Raider will never be considered a serious FPS game, it's more a puzzle game and frankly quite boring ;|
Left 4 Dead - Not out yet, someone else mentioned there's only two levels, and one game mode. This is because it's a DEMO. It is in fact one level, split into two parts. If your son is looking for an FPS-Action game playable both solo and co-op (whether it be with friends or random via the server browser), I'd recommend L4D based on my experiences with the demo.
Call Of Duty: World At War Never been a fan of the CoD's, shooting has always felt a bit awkward and the weapons were not really balanced, I didn't like the movement either, but hey, this could be personal taste, it seems quite a few do like the CoD franchise, I have not played this title myself and do not plan to
Gears Of War I'm going to assume you're talking about the first one, my opinion doesn't really differ between them either way. Despite what people will say, and I'm sure many people here will disagree with me, it's not really an FPS game. Your movement is limited, you have set targets and set strategic vantage points, for me it felt like the old Time Crysis games, without the manual shooting. Graphics are great, as are sound effects, especially voice acting, the game also features co-op which I don't have much experience with, but it's not a game I played that much.
In short: Left 4 Dead or Dead Space.