Asking 43 lb. person to carry 20 lbs. is the same as a 135 lb. person (like me) carrying 60 lbs and there is no way I could do that, at least for very long. There is a reason this site exists and that is because few of us are in the condition of a Special Forces or Seal team member and I would bet your kid isn't either.

Keep it light, keep it simple, keep it fun. Your kid isn't training for special ops, so don't treat him like he is.