My favorite game is GTA, but he might be a little too young for it. I would just go with racing games and maybe some shooter...if he likes shooting games. Some games I like are Assassins Creed, Call of Duty, and Gran Turismo. Little Big Planet is a great game as well (had it on my PS3)
Being an XBox family, lol, I don't know the names of the PSP games, but I would also chooses a couple of the "game" or "Learning" type of games like if they have a Momoply, Trivial Pursuit, etc. That is something you might even want to play with him and cut down on the "head stuck in the game" isolation you are getting ready to start seeing, lol! (I raised two boys in the Nintendo and Mario Brothers age. Legen of Zelda rocked!!!..."