The Wii

Yeah, this is fun. Except for when you are jumped upon all the time.

I AM NOT YOUR YOSHI.

Tell me about it.

Mario Kart Wii - Great game for the whole family
Zelda: Twilight Princess - Great game overall
Super Paper Mario - Good, collector-esque game
Excite Trucks - Not the "best" game; it doesn't have a real plot and gets...frustrating, but is a really addicting game none-the-less.
Rock Band - If you don't already have it for another system, a fun game that the whole family can get into
Star Wars: Republic Heroes or Star Wars: Force Unleashed - Star Wars games are good for the Wii, with their lightsaber movements. If you're son is in to that kind of thing, that is. Star Wars: Republic Heroes is a little more kid-oriented, though.
Mario Strikers Charged - Fun when played with multiple people
Mario Party 8 - Another family oriented game

Pretty much what others have said.