Recommend going to an REI or similar outdoor store and have him try packs out to fit his torso, etc. When you get a better idea what "types" of packs he feels fit him comfortably, you'll be better informed to peruse Mystery Ranch (primarily to carry game or camp?), Mountain Hardwear, Marmot, Black Diamond, Sacs Millet, etc., etc. to potentially order. I would ask the store to put a 10lb weight in the pack when trying them on to get a better idea of its suitability.
Get some "experienced" assistance at the store to ACTUALLY and PROPERLY adjust the pack while your son wears it!
Get something that will fit him for a couple of years and THEN get another pack later. Packs are like rifles and revolvers...you can always find a use for another. Most quality packs can reduce the volume of the pack bag AND adjust torso length so a bag you buy now may last for many (several) years as he grows. Obviously, buying a Mystery Ranch pack that goes for $500 may make that a future-proofing issue.
Recommend buying NOTHING over 4lbs. Lots of features usually result in more weight. Most packs allow removal of the pack lid and sometimes other components to help keep weights down.
In my region, waterproofness is a big factor...may not be important in your area.
