Is playing wii fit good exercise for my 11 ye old son?

Wii fit: Careful. There is a lot of joint tendinitis upper/lower extremities and shoulder because of overuse, improper mechanics, lack of proper warm ups. There is no substitute for actual outdoors exercise. The wii yoga/strech is pretty good (adults more likely to use it than kids).

Yes: But make sure , as others have mentioned, that the mechanics are proper and it is done with the right level of moderation.