I would keep him in the 20 if he's eating ok for you--if not, you may need to move him down to a 10, or a small plastic tub. Definitely save the 40 until he's fully grown, (if you use it at all).
Take care that the bedding is not wet--wet substrate allows rapid growth of bacteria, and quickly leads to dangerous skin infections. It is a challenge to raise the humidity while keeping the substrate dry, but that is what they need. Consider a cage humidifier, or seal most of the screen top with plastic wrap.