To start out: I do NOT own a 360 (but I have no reason to do so).
PS3 vs. Wii
PS3:
Hardware: High Definition Gaming (on HDTV), Free online play from sony, Blu-Ray movie support, quiet, USB ports, Wi-fi capabilities, nice controllers, graphics are still very good on standard TV, comes with 120 GB HDD
Games: HD gaming, action games are great on HDTV, almost all major games are available (no Halo!), great exclusives (Metal Gear Solid!), games have more content than their respective Wii versions
Cost: $299 + $60/ new game
Wii:
Hardware: Standard Definition gaming, motion-based controllers (can get your arms sore after playing for a couple of hours), supports GameCube games and controllers, Wi-fi capabilities
Games: Usually meant for younger kids or early-stage gamers, have Mario games, games are generally shorter than their PS3 equivalent titles, game selection not that great
Cost: $199 + $50/new game
If you are not a gamer and would like to partake in your kid's gaming experiences, I would get a Wii for now. You can always get a PS3(or 4) later when he will care more about graphics and gameplay --- the Wii will satisfy a 9 year old.
If you like to play games and would like your kid to join you for some real gaming then a PS3 is the way to go. He can learn to use the PS3 easily, especially if you are comfortable with the device.
Personally, I find the Wii to be losing ground with their bland gaming selection. Nonetheless, 4 player Mario Kart and Sonic Olympics on the Wii are always fun when friends come over. I love what the PS3 offers despite the higher game price, these games are worth it and the selection is GREAT.
Good luck and I hope this helps.