How much does he want to spend? Does he want something that will plug in and work, or something he can tinker with? Does he want to play games, or use it purely for media playback? If he wants to play games, what kind of games?
My preference (and hence, what I've gone with) is a PC - but I've kitted it out with a quad-core processor and a 6970 graphics card so that I can still play some games on it too.