All iguanas get change-related stress from being shipped and introduced into new surroundings. It can temporarily inhibit eating, drinking, and growth.
So we can assume he eats his food? Has he shed yet? Have you seen him drink? (If he eats his food, he can drink less water.)
One thing that can easily go wrong are the temperatures. When we use thermometers in the basking zone, they have to be placed in the tank. Then they absorb heat and give falsely high readings. If you get a non-contact thermometer, you can directly measure your iguana's skin temperature. It should be at least 95 in the basking zone, and 70-80 or so in the cool side of the tank. You may need a bigger terrarium to get that temperature spread. Iguanas instinctively move from cool to warm, and back again, self-regulating their body temperatures.
Proper temperatures are needed for digestion and proper immune function.
You can get non-contact thermometers at Radio Shack, Harbor Freight, and automobile parts stores, and on-line, for about $30.00.