The Sanitas is over 4. I would probably not recommend that one.
But, there are other packs out there. Lets find some:
For all the external haters....here are some internals-
REI Passage 40
This one is 3 lbs 4 oz and $99.

Deuter Fox 30
This one is 2 lbs 7 oz and $100.

Deuter Fox 40
This one is 2 lbs 14 oz and $110.

Osprey Jib 35
This one is 3 lbs 3 oz and $130. It also says it is sized for 10+ year olds.

All these packs are lighter than the Kelty Jr Tioga. The Osprey is lighter by 3 oz and the REI one is lighter by 2 oz. I would take the Osprey off the list, just because I don't think it would fit. The Deuters look like the best ones. BUT, only if he has a proper sleeping bag, and the pack fits the kid.
The OP asked where the best place to start dropping weight is. I would say that the sleeping bag is the best place. The sleeping bag determines the rest of your gear. A light compressible bag can fit in a small light pack. A big bulky bag needs a different pack.

My advice, if the kid is less than 7, only make him carry a frameless pack (book bag) with rain gear, snacks and one liter of water. 8-10, keep his pack weight less than 10 lbs total. After 10, slowly increase the weight, depending on his growth and strength. At 12, he can carry his own gear. At 15 or so, you make him carry your gear to slow him down.