I am not familiar with these packs--I've never been able to wear an Osprey pack because their hip belts all cause my hips considerable pain (just to illustrate that pack fit is almost as individual as shoe fit).

However, I'm very familiar with teenage boys, having raised three of them ahd then having four grandsons. Your boy is at what is usually the most rapid growth stage, so you may want to wait a new pack until closer to the event--or you may be buying another one by next summer! Of course if his present pack is already too small, you may have to bite the bullet.

Pack measurement has little to do with height, but depends on torso length--the length between that bump at the base of your neck and a point on your spine level with the top of your pelvic girdle. That measurement doesn't necessarily correspond with height. Lots more info about fitting packs on the home page of this site.

It's also a good idea to have your gear, or most of it, handy when trying on a pack. Real life gear has quite different weight and bulk than do sandbags or other weights. Be sure to include a mockup that approximates the weight and bulk of a week's food. If you can't haul the gear to the store (good idea to make an appointment first), do this as soon as you get the pack home, while the tags are still on and you can return it.