S.
We have Wii, Playstation 2, psp and nintendo ds. It depends on what he is into and how you want to use the system. Between the playstation and the Wii, I prefer the Wii. But my kids play both of them equally. If they want to jump around and play, they use the Wii. If they just want to play a game quietly, they use the playstation.
Psp and Nintendo Ds are handhelds and are a blessing if you have other children in activities. They are really small and portable and are great in the car, at sports practices and other places not at home. whatever games that you can play on a regular playstation, you can also play on psp (you have to buy boht the psp and the playstation version). The Ds has more educational type games - brain age and big brain academy are great for school.
The decision is really what you want to spend, how many kids are going to be playing, what type of games he likes to play and if you are looking for portability. Also, just to let you know, my kids are 12, 10, 9 and 5. My 5 yr old is getting a DS from Santa this year! My girls like the Wii and DS and my boys like the PSP and the playstation.