I went through exactly the same frustration when my grandchildren became old enough to backpack. Kids' backpacks all seem to be made with bombproof super-heavyweight nylon, with lots and lots of pockets, really heavy zippers and lots of basically decorative junk. When the pack itself weighs half of the recommended weight for the child–not because of its structure but because of the gewgaws–that's equivalent of a 15-lb. empty pack for an adult carrying 30 lbs.! To me, that's unconscionable! If they would make just a simple nylon pack bag (like the MLD Kids' Prophet) on the frame of a Deuter Fox, you'd have a really great pack for a kid, probably about 1 1/4 to 1 1/2 pounds. If you have sewing skills (I don't), you could try doing this. You'd have to throw the Deuter packbag away, so this process would be less expensive if you could find a used one. In fact, unless your son has a younger sibling to inherit the pack when it's outgrown, you should look for a used pack anyway. Depending on how fast he grows, whatever you get for him now (or next year) will be outgrown in 2-3 years, anyway–an argument for finding a used pack.
I agree with Sarah that kids need good structure in their packs. A 170-lb. adult man can easily carry 15 lbs. total pack weight in a frameless pack, without a hipbelt–it's less than 9% of his body weight. The same pack on a 45-lb. six-year-old would be 33% of his body weight. Put another way, that 15 lbs. on an adult would be 4 lbs. on a 45-lb. kid. If the 170-lb. man is going to carry much more than 15 lbs., he will undoubtedly want more structure in his pack to remain comfortable.
The big complaint of kids with packs without hipbelts and a good weight-transferring structure is the shoulder straps' cutting into the tops of their shoulders. My first backpack was at age 6, back in the days when no packs had hipbelts (adults used a "tump line" around their foreheads to help support the weight) and my pack didn't even have padded straps. I still remember vividly my sore shoulders, with total pack weight of 5 lbs.! My eldest grandson complained of the same thing as a 5-year-old, with a 4.5 lb. total pack weight and well-padded shoulder straps.
Unless your son is unusually big for his age, I don't think you are going to find a backpack (as opposed to a daypack) that fits him, They're going to be a little too big and therefore probably hang from his shoulders. He will probably grow into the Deuter Fox in another year. My grandkids used daypacks at ages 5 and 6, which was enough for their extra clothing, rain gear, snacks, water bottle and a toy or two. This brought total pack weight up to 5-6 pounds, which is probably all you want for a beginning 6-year-old anyway. Remember that you want his early backpacking experiences to be fun for him. Get him hooked on backpacking, first, and load him up later. Unfortunately, that means loading you up, but the first trips should be only a few miles anyway.
However, finding even a daypack that is supportive and fits the age 4-8 set is a major undertaking. Most school packs are too big for young kids (they hang down well below their buttocks) and don't have hip belts. I finally found a really good one, the Granite Gear Sidekick, which has well-padded shoulder straps, a partly padded hipbelt and a weight transfer structure (foam pad) that is good enough for 5-6 lbs. It has been used by two grandkids and will be inherited by grandchild #3 who just turned 4. It isn't big enough to hold a sleeping bag (at least not a synthetic bag) but will comfortably hold everything I described above. Now the really bad news: Granite Gear has discontinued that pack! You might check on ebay or various forums to see if anyone has a used one for sale. I considered Tatonka and at the Tough Traveler Super Padre, but ended up with the Sidekick. I noticed that the Tatonka Mowgli has the option of a pirate pattern–would your boy be comfortable with that? You could always remove the teddy bear.
Last summer, when Gossamer Gear was clearing out their Mariposa backpacks, I bought two small ones. My grandkids (now 7 and 9) will be getting them for Christmas. The small size in the older model (not the newer one, unfortunately) fits down to 12" torso length. WIth the stays and the stiff pad in the back, they are plenty supportive. In fact, I'd have gotten one for me except that they are a little too narrow in the shoulders (which is good, for the kids). The packs weigh 18 oz. and should last until they grow into adult size. Unfortunately, their capacity is a little too big, but the small size Miniposas were gone by the time I discovered the sale. They can be compressed a lot smaller with the provided bungee cords. 7-year-old "little" sister is as big as and a better athlete than her older brother, so she'll be big enough for the new pack. In fact, I'll have to give her as much weight as her brother (will be about 8-9 lbs. with sleeping bag and pad added) so he can keep up with her! As you can see, a lot depends on the individual child! Again, this pack won't help you until a couple of years later, since the 2009 Gossamer Gear packs will start at 13" torso length in the small size.
A couple of good articles on backpacking with kids, although not much help with gear:
http://www.backpacking.net/kids.html (skip past the part about babies)
http://www.the-ultralight-site.com/backpacking-with-children.html
A couple of things: Unless your boy has been dry at night for several years, I'd really hesitate to use a down bag. The stress and excitement of being out in the woods is liable to bring on middle-of-the-night accidents (I speak from bitter experience here!) and/or digestive upsets (more experience!). Second, I've found that the little things are often more interesting to the kids than scenery. One one trip last summer, following a 6-week unit on insects in 2nd grade, my grandson (and "little" sister) were basically uninterested in any wildlife larger than a beetle, but they studied insects for hours! They did get distracted by one of our oversized Pacific Northwest slugs, but quickly returned to observing an anthill. Finally, I agree with those who prefer the cut-down Z-rest, Ridgerest or similar foam pad. Lots lighter, and kids don't need the inflatable. Thermarests don't take too well to being bounced on!
