So, hopefully by the time your son reaches 40 pounds he’ll be able to go hiking himself. 
When my son was little he always asked me to pick him up; I noticed that the heavier he grew, the stronger my muscles got. Keep bringing your son hiking with you and you’ll be in good shape to carry him.
Anything above 50 lbs is a heckuva lot of backpack. Even that isn’t too bad once you get it settled just right, but swinging it onto your back and buckling up is a major enterprise; balance issues are more significant than with smaller poundage; and you’ll end up with holes in your skin at abrasion points where otherwise just some mildy sore / reddened areas.
Also not all packs can hold up with 50 -70 lbs in them, you might get 5 miles down the trail and find out your pack is falling apart.
That’s why I don’t have one of those.
Look, people have been carrying heavy loads on major trips for a long time. If you can get away with carrying less, then great, but it’s not always possible. On most weekend or overnight summer trips my pack can easily be under 25 lbs, but not always. My winter daypack weighs more then that.
I’m an out-of-shape 53-year-old who went on a backpacking trip last summer with a pack that was just about the same weight as you describe – 30 lb. (13-14 kg). I managed it, but it wasn’t much fun (the hiking part, that is).
I am going to Nepal this year and I don’t care what my pack weighs. Because someone else will carry it for me. I will have a daypack too.
I’m sure those $500 Arcteryx packs can handle 70 lbs with no problems. I just don’t want to pay that for a pack.
I can’t carry more than 5 pounds or my arms start to go numb, even if the armstraps are just hanging loosely as per above post 
Fortunately it’s not that bad if I carry the absolutely lightest backpack, tent, and sleeping bag less than ~$200, and that only comes to around 10 pounds (I weighed 'em.)
If I ever go backpacking again I’m going to carry most of the stuff on loops on my hips instead of my backpack. And preferably only on trails with shelters so I will only have to carry an (even tinier) emergency tent rather than the individual tent that can only fit me without space to even sit up which still weighs more than 3 lbs.
If you trek in Nepal you can get by without a tent or food or maybe a sleeping bag , you stay in small hotels. You just need to carry clothes, camera, and other small stuff.
My pack in Nepal for a 4 week trek was only slightly larger then my normal daypack.
We shouldn’t forget to account for the level of fit/finish and design of packs. If one has a well-engineered pack one can comfortably carry much more gear than otherwise. The difference (design and materials) between my two packs, both purchased for $450 (one in 1993, one in 2007) is remarkable.
It’s completely unrelated, but I use my pack for shopping and travel. I don’t walk a long distance with it when shopping, but I routinely carry 100 lbs the couple of blocks to the bus stop. The hard part is getting up the stairs when I get home to our second floor apartment.
It’s an external frame pack, and I have a second, insulated pack mounted below the main pack on the frame.
I have no idea why external frame packs have become so scarce. They are much more flexible than internal ones as far as I’m concerned. I strap all sort of stuff to this one. It’s also my main luggage for my commute from Chicago to Kansas City via Amtrak.
I use my backpack for travel, as well. It’s the largest size you can stuff into an overhead compartment when it’s almost full. If I had the equivalent volume in a hard suitcase or briefcase I’d certainly have to check them in (yeah, when it’s almost full it exactly fits into those “will your bag fit?” slots with minimal coaxing from my hands.)
I think internal frames just became the “in” thing to buy. They probably are easier to balance since they hug your body. External frame are probably harder to put on a plane too.
Internal frame packs are easier to brace gear and secure tightly, making them more desirable for hiking on inclines. Hiking with gear dangling off every which way might be okay for backpacking on level ground but it is really a bad idea on narrow mountain trails, and especially off-trail rockhopping or climbing. Ex-frames also have the nasty tendency to snag on underbrush or other luggage, and the external frame adds more weight than an in-frame pack of the same capacity. Ex-frame rigs are still useful for humping really large or bulky loads, but in general internal frames are far more convenient and well-balanced.
Stranger
Ignorance fought, thanks.
I’ve owned internal frame packs, and lusted after an external so I could add the second pack for frozen food. I shop at CostCo, and will fill both packs up and strap a 36-roll pack of toilet paper to the very top. The sight of this brings laughter and joy to the other people on the bus, so I figure I’ve done my part to make the world a better place.
You can find mil-surplus LC-1 ALICE pack frames pretty cheaply, and they function well as general purpose gear carrying appliances, especially with the cargo shelf attachment and some general purpose nylon webbing. Do not buy MOLLE frames (junk) for heavy cargo hauling.
I’ve owned a couple of ex-frame packs when I was younger, and while they were fine for relatively short hikes (<10 miles) on level ground, the in-frame Osprey and Arc’Teryx packs I now use are vastly superior for general purpose hiking. I’ve actually done semi-technical climbing and repelling with in-frame packs, which is something I would never do with an ex-frame pack.
Stranger
When I used to do a lot of backpacking I would typically carry around 40 lbs (weighing 170 lbs). Once, while doing a backpacking trip into the Needles district of Canyonlands, I had a pack that weighed about 75-80 lbs due to the water we were carrying in (the only available campsites were in the grabens where there was no water). Even though I was young and in pretty good shape at this point in my life, I was wiped out and feel I came pretty close to doing some serious damage to my body (mostly knees)…
Something’s not right either in how the pack fits you or how you’re holding your body.
If you’re hiking from shelter to shelter you don’t need a tent at all - most have ‘shelves’ to sleep on off the ground and if bugs are your concern, you can just buy one of the mosquito-netting hats. I’m not sure what you mean by ‘emergency tent’, but a bivy like this will do on it’s own in mild weather. Obviously, if you’re going in the winter or expecting big storms you may want something more or at least higher-quality, but a friend of mine uses that exact model all the time in the spring and summer here and swears by it.
You can’t count on getting space in a shelter, you should always carry some form of shelter (tarp, bivy, tent) for your own use. It’s common in the Northeast for folks to plan on staying at one of the popular shelters only to find them full and all the official tent spots taken too. We have a lot of hikers here.
I’ve never heard of a park where you don’t need to make advanced reservations for a shelter, nor one where you’re allowed to pitch a tent. Are these areas where you need a backcountry permit?