Currently, the PS2 is going to go under a revision pretty soon. The ethernet adapter will be built in and the dvd player will be updated, which is a major plus, and a few more additions. (mine is going out and it wasnt that good to begin with for dvd movies, games just fine though) You can get a upgrade kit that lets you have a hard drive and will let you install linux.
XBox. Currently the most powerful system out right now and has many more modifications that you can do to it. Install linux, turn it into a media center pc, upgrade hardrive and save games to it (no load time then), etc.
I have a ps2. I love the joysticks and that was my deciding factor going up from a ps1. I cant stand the xbox controllers, but there are others out on the market.
The games really are pretty much the same on both systems. They eventually come out for both, with the exception of doom 3, ps2 probably cant handle it.
If he wants multiplayer games, both systems can do them. Although, many game companies do not like xbox live as they do not make much off of it. Sony supplied a hardware adapter and it is up to the game company to decide how long they want to offer a free subscription to their game. I think madden 2003 has a one year subscription. I dont have this, but i have read a bit about it.
Dont go for gamecube. It is a dying console and nintendo is having trouble putting out enough games for it that arent geared towards kids.( Its primary market is kids up to 14 whereas the xbox and ps2 are geared towards an older crowd.