I would suggest changing his food. Carrots and romaine lettuce is not good for iguanas on a regular basis and will cause major health problems, I would suggest you stay away from them all together, there really is no nutrional value in any lettuce and very little calcium. Use collard green, mustard green, kale, parsley, spinach, and cilantro more. Use collards and mustard greens as his base food. These are high in calcium and do not cause the problems that other greens do. Spinach use sparingly as it can cause a loss of calcium in iguanas. A good rule of thumb with your iguana is always have 5 different forms of veges with each meal. I usually take base foods and mix them with small amounts of the others I have listed. Check out this website for great info on iguanas
www.anapsid.org
If you are concerned of his not eating than I change his substrate to newspaper so you can see if he is defecating. Feed him at the same time everyday, also when iguanas are young they do not have the body that adults do, they are usually skinny, but they are also far more active than the adults.
I have two adults currently, one was caught in the wild, she is very skinny, but she eats and is healthy. The other is domestic and is a fat girl. I have noticed this between wild and domestic iguanas that my friend owns with his rescue(he has a lot of igs), there is no written documents to back me, though it is something I have kept my eye on and is my own beliefs.
How long have you had your iguana?