I'd recommend the following order
NDS > Wii > PSP > Xbox 360 > PS3.
The NDS can be used everywhere and isn't as violent as PSP and other games. The Wii promotes multiple people playing for the whole family: get Wii Sports, Wario ware, Mario Party, Super monkey puzzle, etc. The PSP games aren't that great, or just very difficult for normal people, but cheaper glam factor than the Xbox/PS3. The Xbox 360 has simply been available the longest and hence has the most games, but has hardware issues according to the news which might mean repairs. The PS3 is huge, expensive, and limited games, with a controller worse than PS2, but the games do really look pretty, and they might be releasing a new controller soon.
PS3 is heavily discounted in stores in HK to presumably Sony trying to stop Japan imports, the Wii is generally available in computer markets because there are only US or Japan versions.
The PS3 really isn't selling, especially in the computer markets. Even though there is a Gundam game - which is very popular here. But there must be someone buying it, you should ask what your sons friends have, maybe they like the games that are available.