That makes it a little bit of a tougher decision - since he's an adult (ish), some of the games that he wants are probably going to have additional DLC, map packs, that sort of thing. I just got Forza 4 and to get access to all of the cars requires 2.7GB of space - which would eat up almost an entire 4GB memory unit.
However, there is an alternative - the Xbox also allows the use of USB drives as memory - You can go and buy a 16GB stick for under $20 now and I'm sure that would be sufficient for everything there. I'd say if you're just looking at exclusives, grab a 4GB unit and a USB drive. If you want to upgrade to the 250GB HD, that option is always available to you.