Welcome to the forum!

You don't want to put too much weight on your son for quite a while. Keep it light! Clothes and his sleeping bag (if light) are all he's going to be able to carry for a while, and unless he's unusually big and strong for his age, I would keep his load down to about 7 lbs. He needs a good supportive pack, too. Unfortunately, nearly all childrens' backpacks weigh 2.5-3 lbs., so the pack itself will make up about half of what he can carry. You want to make it fun for him so he's willing to go again!

I'd suggest getting together all your other gear before pack shopping, so you know the pack fits the gear (and vice versa) and that you can carry it comfortably. My son does something similar for our annual trip (he, 2 kids and I--Granny) to the Olympic NP beach; he is a surfer so has his wet suit (really heavy, especially when wet) and surfboard in addition to all the food, the 3-person tent and his bedding. In the past he and I have split the kids' stuff, but this year they'll be able to carry their pads and sleeping bags as well as clothing. For what it's worth, my son has an Osprey 85 pack and finds it very comfortable even with 40 or so lbs. (it's rated for 50-55 lbs.).

The other thing you need to do is to keep all the gear as light as possible. I'd seriously consider a lighter weight tent, such as a Tarptent Rainshadow. .

If you haven't read the articles listed in the left column on the home page of this site ("TLB Home"), I suggest you do so. They were responsible for my being able to cut my pack weight in half, so I could continue backpacking in spite of a bum knee and general old age.

I really admire your wife for being having the courage to get out despite her disability, and both of you for getting your son out backpacking!