The very first post in this thread will answer most of your questions.
If he can use a DS with success then I believe he'll be fine with the Wii. Just harp on the importance of keeping control of the Wii-mote. There is a wrist strap that you will want to make sure he always wears.
The system is $249 and comes with everything you need to get started including a game and one controller. For more accessory details, please read the FAQ!
By Christmas you should be able to walk into a store and find one. if you do decide to get one, I suggest picking it up around October, by November they'll probably be scarce again as everyone tries to get them.
Think of the Wii as a Game cube with a special controller. It's a big deal, because the controller is so new and different than ever before and it gets you up and moving unlike traditional games.
The big deal with Mario Party 8 is that it's the best Wii game to come out for a while. We've all been waiting for a new game to buy. MP is awesome for families, but not for just one player, so mom and dad will have to play too. I would suggest picking up one of the other 7 Mario Party games if he's never played one before (6 or 7 preferably). They're all on the Cube.
The good news is he can play all of his GC games on the Wii if you decide to get one.
Hope this and the FAQ can answer your questions, but please feel free to ask if you have more, that's what we're here for!