All the proper kids games are already on the PS2 and PC, you dont need to touch Nintendo in that respect, he could just expand his collection to use on your PS3 or PC.
You dont mention what type of games he likes, but Nintendo's style is mostly quirky Japanese action adventure or puzzle games, with bright colours and simple graphics, that you either get or dont (most people fob it off as for kids only which is totally wrong) The games tend to ooze quality the longer you play them, again not something kids always appreciate.
The Wii at them moment is for party games and family entertainment that gets you more involved with your kids development as opposed to letting them shut themselves in their bedroom.
Having a round of Golf or some frames of bowling with them standing in your front room is the next best thing to taking them down the park. Sadly my younger kids <5 find the controllers too sensitive and fiddly.
I would look at the Nintendo DS to see what sort of games can be expected for the Wii, as the Wii doesnt have a big library at the moment.
I think there will be some good adventure games RPGs, and Sim games.
Wii Sports and Wii Play are must have party games, Wii sports can be taken seriously despite cartoon graphics, has some good training modes.
Mario Strikers (not like real football, some kids might hate it, some adults will love it)
Excite Truck (I liked the idea, but neither me or the kids got excited about it despite the controls method actually being quiet good)
Mercury Meltdown (good game, very addictive, again not all kids will like it)
Theres several Mario games on the way, that look childish but are likely to be very involving adventures, these are the sort of quality games kids dont forget and the original Playstation generation missed out on.
Then theres some Sega exclusives, that could be worth a Wii in its own right, Mario and Sonic Olympics (if its not a wind up) and Nights, to name a few.
For yourself dont forget Resident Evil 4 (for adults only but one of the best Wii games, better than the PS2 version, more involving with the controllers and not bad graphics either, a better game than many technically superior PC FPS)
Just dont expect too much of the Wii at the moment, and dont take it for granted that kids will like it.
Its also probably the best platform for retro gaming, at £4-£6 for quality older titles you can play with the excellent Gamecube wavebird controllers.
