If you don't mind butchering creatively trimming the REI Comet, you can get over half a pound off it. I removed the lid, the useless heavy bottom zipper (I sewed the gap closed by hand) and the various decorative gewgaws. Admittedly, the one I whacked at was a discontinued model from the REI Outlet which was only $30.

A few of the "cottage" manufacturers are expanding into the youth pack market, although I suspect your boy will have to grow a little more first. ULA has a youth version of their Circuit backpack with an adjustable torso length, 12" to 17". It's an internal frame pack. The Gossamer Gear Mariposa in size small fits 13" to 16" torso length. It has a removable stay which should be left in for better support and basically makes the pack into an internal frame pack. The Elemental Horizons Kalais is another pack that can come in youth size, and the owner will do some customizing for you. It also has a removable stay (extra) and of course for youth it should be used with the stay for better support. None of the three are particularly cheap (range from $190 to $235 plus shipping), but they are lighter than most other youth packs. If your boy is skinny like my grandkids, the hip belt size may be more of an issue. I haven't tried any of these packs for my grandkids yet. The oldest (13 next spring) has about outgrown his Deuter Fox 30, and I may get him the ULA Circuit if I can persuade his parents to help with the price. Of course sometime in the next few years we can expect him to outgrow everything.... It helps, though, that he has two younger siblings to inherit what he has outgrown.