This isn't going to help dollar-wise, but ULA has a youth version of their Circuit with an adjustable torso length up to 17". This might keep the young man going for years, well into adulthood.

If you want to do some butchery creative remodeling, you can get a pound off either the REI kids' backpack or the Deuter Fox. Of course unless you're better at this process than I am, the kid may object! There's also the Osprey Jib but it's a lot more expensive. Watch for sales or used packs (it's kind of hard to start whacking deliberately at a brand new $100 pack!). If you're really creative, you can make a whole new pack bag out of lighter material, using just the frame and support system of the above packs, which would save even more weight.

There are of course lightweight frameless packs, but the kid is going to need some support to carry weight. And most of those will be too big, since (except for ULA) the cottage folks don't make kids' packs.

If he's going into Scouts, enlist the help of the leaders! I wouldn't be surprised if they know of outgrown packs ready to hand down! Probably other gear, too.