You guys..jeez. Where's the love?
Anyway. I hated Mag but you're here to talk exclusives so if you can get it cheap, why not? The backdrop for Resistance is kind of cool and number 1 and 3 both have a "weapon wheel" so you get the pleasure of carrying a thousand pounds worth of weaponry and it really does make them more fun than 2..which went with the lame ass Halo two weapons only setup.
Ratchet and Clank will blow your nephews mind. My daughters both love them and they're not really "gamer" girls. The series is made by Insomniac, who also did Resistance, so it's like the crazy weapons from Resistance cranked up to 11. And though it didn't get any really amazing reviews Ratchet and Clank: All 4 One has four player co op and it's a lot of fun with your kids..or in your case nephew to tool around in a Ratchet and Clank "lite" environment.
The whole Uncharted series is great. Pretty solid mechanics though guys take a few more shots than reality would like to argue. AND it might be one of the only franchises that won't be bashed to shit on the boards for it's portrayal of women...I said might.
Demon's Souls is one of my favorite games of all time. Don't think the hype is light though, it isn't what I would call a casual game so much as being water boarded and thanking From Software for the pleasure.
I love the Metal Gear series to death but 4 is the epitome of, "Wow this game is great..I'm sneaking around and it looks really cool..AH! Giant robot thing that moos like a cow!..oh okay this guy is talking to me *ten minutes later*..Man I have to pee...*go to pee*...*come back still talking*..Hmm, this guy is kind of a dick..I'm hungry..*go make a pizza only to comeback and they're still talking*."
The God of War collection and the Sly Cooper collection are no brainers. They're both a lot of fun and Kratos, from God of War, is the poster child for rage quits. Not in the sense that the game is too hard but he just rage quits on life in general. All bro.
Your nephew might dig Katamari Forever too. It's /very/ japanese but pretty funny and definitely kid friendly.
3d Dot Game Heroes is great. Buy it.
Socom is boring to me but you might dig it, I kind of doubt it though.
Modnation Racers is another game your nephew might be into if he likes racing.
Gran Turismo, meh. not my thing but they all look really, really pretty.
Heavy Rain is more of a "movie" than a game but I enjoyed it quite a bit. It's got a slow burn feel to it. "JAAAAASON!"
Ico and Shadow of the Colossus are obvious buys. I like Shadow more, but they're both great.
Feel free to tell me how dumb my choices are or how you hate them, etc. That'll really help Rain figure out what he'll enjoy.