I wouldn't be so concerned with the overall weight of the pack. I mean, unless it's obnoxiously heavy, a pack's weight is the least important of it's virtues. Of course you'll need to be looking only at packs that will carry all your stuff, and for now, most of your son's gear as well. So once you decide on an appropriate volume, it's pretty easy to narrow down your choices.
First, it has to fit your torso. There are a couple of ways to measure your torso accurately. Try every method you can find and average the results. Some packs are adjustable, but make sure the load-lifters can make a downward angle to the shoulder straps, even at the longest torso length.
Second, the hipbelt needs to support all the weight of your pack full of gear, without loosening or riding down on you. The shoulder straps should only function to pull the pack tight against your back.
Third, it needs to have the right mix of features for your routines. How many exterior pockets do you want? Do you need a mesh panel to put damp clothes in to dry? Bottles or hydration bladder?
For me, my favorite pack for what I do is the REI Pinnacle 35. It's heavy! Designed as a rock-climbing bag, it's made of tough HT nylon with a thick plastic frame sheet and aluminium stays. In its stock form it weighs over three and a half pounds. I knocked half a pound out of it by cutting off gear loops, climbing-rope tie-downs, and superfluous buckles and clips. It's still a three pound pack, but it fits me, has a great hipbelt, carries my gear close to my back, and it's bombproof.
So what I'm getting at, is don't rule out slightly heavier options, as they might make up for the weight with superior comfort.