Fable 3 is a good idea, I agree. Not very complicated, just mainly hack and slash with a litle side story to keep things interesting. Fallout 3 and New Vegas can be fun games to roam around in blowing stuff up. Grand Theft Auto series are probably right up his sleave as well (Although I personally enjoyed GTA III and Vice city more than the newer ones, but their graphics are a bit outdated now).
A little bit more complicated, but as it turns out not overly so now that I've finished it, is Skyrim. I'm personally a bit let down with the game as I epected more depth to the main story line, since I personally enjoy story, depth and challenge more. It's a great fantasy world to roam around in though.
The upcoming diablo game might be for him as well, but it might still take a while for that to get released and I've no idea if it goes to the XBOX.