If you're looking at a Swiss Army knife, which for that age has a lot of cool factor, I'd look at either the
Spartan /
Tinker or the
Recruit, depending on his hand size. The Recruit is a bit shorter at 3.3" and just lacks the back tools (which I don't see as a huge drawback). Or you could go up to tools with scissors or a saw, or both (
Climber/
Super Tinker,
Camper/
Hiker,
Huntsman/
Fieldmaster). I'd lean toward the models with screwdrivers vs. those with corkscrews. I don't imagine a 9 year old would have much use for a corkscrew for its intended purpose.
The four layer knives have a good tool selection, but might be a bit much to slip into a pocket. I'd stick to 3 layer or less. The Alox versions (
Pioneer,
Farmer,
Pioneer X) will be slimmer and have somewhat better build quality, but are more expensive.
Note on the links: Those all go to Victorinox's website. Although they have "buy" buttons, they're not affiliate links. In fact, I recommend against buying direct, as you'll find better deals on other websites. (Knifecenter, who I've had good luck with; BladeHQ who seems to have a good rep; and Amazon though you need to inspect the product for defects since they just resell returns and also need to make sure Amazon is the seller, since some of the non-Amazon sellers may sell counterfeits.)