Along the lines of what Chris said, a backpack is something that you'll need to try out in person before buying. I'd suggest you find a local high-quality dependable outdoor shop. REI is a national chain that has a very good reputation, but there are lots of local gear stores around the country.

Here is the advice my Scout troop offers new boys regarding backpacks:

When you decide to buy a backpack, get an external frame backpack that fits your son. Firmly resist buying oversize! The pack volume should have about 3000 cubic inches. Make sure it has a padded hip belt, padded shoulder straps and support webbing across the back. Don't buy an internal frame "mountaineering" pack; they're hotter than blazes (cause they lay flat on your back) and the lack of an outside frame makes strapping on sleeping bags, foam pads and tents a royal pain in the behind. Yes, they feel very comfortable in the store and the salesperson will rhapsodize eloquently over how marvelous they are - Don't!

Avoid using a backpack more than five years old, since newer designs can make a big difference in comfort. The more side compartments on the bag, the better - they help your son keep things separated and organized. This is one of those items that needs to be tried on before purchasing. Brands to look for: Camp Trails, JanSport, REI, Kelty, Dana Designs, Gregory, Mountainsmith, The North Face, Osprey, Arc'Teryx, Granite Gear, Coleman Exponent, and Lowe Alpine.