I agree with BZH, you could get a used climber's day pack and use a cut down folded foam pad as a stiffener. Your son probably doesn't need a full pad and a foam one is @ 12 bucks.
Some of the climbers packs are cheap and have a frame of sorts already. I saw a Deuter Speed Lite 30 on sale for 75.00.
A DIY tarp set up with a little velcro bug netting would be cheap, small and light. I would think you could find a small down bag someone's outgrown or upgraded from......