Xbox. No doubt. Better graphics and faster system. Both are online ready. People will argue that PS2 has better games, but that argument is old. Xbox and PS2 have pretty much the same games out. Xbox will not disappoint.
Honestly, I don't think there's much in it. They both have their pros and cons. I own a PS2 and love it and my former roommate had an X-Box which I also enjoyed. For a lot of folks, it comes down to GTA or Halo and depending on how old Jr. is, I don't think he should be playing GTA anyway. Does he not have an opinion?
If you buy him any of those systems, buy a used one. As others have said, the next generation of consoles - Xbox2, PS3, and Revolution (code name for Nintendo's new system) - will be out within the next couple of years.
i have both, but i only bought the ps2 for GTA. i tend to think xbox has better games, unless you're some nerdo who loves rpg's and such. also xbox2 is supposed to come out for the holidays next year.
I bought a new XBox yesterday - 2 games (NCAA 2005 and Top Spin) and 1 controller for $150. And Halo 2 for $45. I'm not going to be studying for a while...