Hmmm... you may want to feed him more. A 43 pound 9 year old? Seriously? How tall is he? My 46 pound 6 year old has always been small for his age.

Anyway, at that age I'd skip over the Tarn 18 for sure since he's sure to hit a big growth spurt any day. Sadly there aren't any lightweight (< 2 lbs) kids packs that are adjustable torso that I know of. As I said, I'll likely end up making my son's next pack. After that one I have a women's small Golite Jam that should work well. That's what I and his older brother use (also for Scouts).