When I do talks and workshops on backpacking for new Scouts, I put an emphases on making the backpack one of the last buys people should make.
Those just starting off in lightweight backpacking should focus first on other gear, like their sleeping bag, shelter and clothing, then choose a pack that best accommodates their gear choices.
My suggestion is to borrow or rent before buying. However, for a younger boy, you may want to consider a pack I recently discovered. It is inexpensive, plenty big for beginning lightweight enthusiasts and can be purchased at Walmart.
Also found at various online stores, including Amazon.com, this lightweight backpack has all the features any Boy Scout would need for most weekend or week-long backpack trips. The best part is that it is 46 liters, weighs about 1.5 pounds and costs about $35!
The Outdoor Products Arrowhead is a small-framed pack, designed for kids. Some adults have purchased this pack and it simply does not fit most people over the age of 13. Based on the reviews I’ve read, the Arrowhead is a decent piece of gear.
To learn more about how to choose a backpack, read my post, Choosing a Backpack.