Are you estimating about 20 pounds of gear for his load, or for your combined loads? The context of the post isn't really clear.

A twenty pound load for a 43-pound kid (I'm estimating he's 7 or 8 years old?) is probably too much for any prolonged hiking (say, more than half a mile.) However, a twenty pound load for the two of you to share...well, are you sure you really need any advice from us?

If the weight and/or fit of his pack is problematic for now, you might consider doing something closer to a car-camping experience. In many of the National Forests in the east, you can drive into the forest, park at a pull-off, and camp near (say, a couple of hundred yards) from the car. You could teach him how to camp, cook, etc., and do dayhikes without heavy packs, all while avoiding the public-campground experience. Then, when he's bigger, you can backpack further in.

Just a thought.