That's why the backpack is the last gear item you should purchase! You need to have a pretty good idea of the bulk and weight of what you're going to put into it before you can even start researching what kind of pack you're going to get. One pack of the same capacity may support 50 lbs. while another may support only 25 lbs.
I don't understand how a pack can have a bad fit if it fits your torso and waist.
As I mentioned, pack fit is as individual as shoe fit. I wear a size 9.5 or 10 B shoe, depending on what the manufacturer thinks that size represents. I don't dare buy shoes without a thorough try-on, even if its a make I've worn before. That's because I have extremely narrow heels and need a wide and high toe box. Each shoe model uses a bit different last, and most manufacturers have a habit of changing them every year or two. The same is true for packs.
You can do the gear trial in the packs at home, as I did, but you need to do it within the period that the vendor allows for returning the pack, which may be only a few weeks. Even REI now is getting really fussy about returns, which they used to accept indefinitely without question. In other words, there's no substitute for waiting until last, or almost last, to buy the packs.
I'm really fussy about food, too; I won't touch stuff like Mountain House, although I do buy ingredients (not meals) from Packit Gourmet, which uses mostly organic ingredients and no preservatives. During the winter, I cook up batches of one-pot meals at home and dehydrate them. On the trail, I add boiling water to the food and let the food stand 15-20 minutes in a cozy--no cooking required. There's lots of info on how to do this at trailcooking.com and lots of recipes there and in the food section of this forum.
With your circumstances, you and your wife will probably have to go shopping one at a time (been there, done that), maybe on different trips, for things that have to be tried on, like packs and shoes. You may want to look at used gear sources, too. Even thrift shops can be a good source--you nevef know when you'll find something good there.
You have my thoughts and prayers for your son!