My son is 9 also and has been shooting for 3 or 4 years now. After starting out with the little plastic bows, he has been shooting a Browning Micro Midas which he's out grown. Now were moving into a Jennings Carbon Exteme Micro. I bought both bows from a guy at work that has kids just a few years older then my son. So that has worked out well. There's plenty of nice bow's for kids on the market.
As nubo stated you might want to check out a bow that can grow with your son. That Bowtech Rascal would be at the top of my list to check out. Plus I would think a Bowtech dealer would set you up properly. If your son get's into something that he doesn't enjoy shooting, he'll probably lose interest.
Once you get him set up check your area for 3D shoots. In SE Michigan most of the places let my son shoot for free, I believe until there 12 or so. It makes the day a little cheaper when I'm paying for myself and my other adult son. Checkout
www.bowsite.com for 3D shoots.
Good Luck