Take a serious look at this bow (link below). It's great for the kids. This one is designed to adjust draw weights and draw lengths so you don't just simply back out the limb bolts.
Draw Weights: 15 - 45 lbs
Draw Lengths: 17 - 28 inches.
It has cam modules that you change out to make the adjustments. This adjustment also changes the draw weight as the lengths increase.Also, you can make some weight adjustment with the limb bolts just like a regular bow.

This one will certainly grow with him until he's ready for an adult bow. I believe it's $250.

http://www.dartonarchery.com/rangerII.htm

-cp