My iguana goes through weeks where he doesn't want to eat a lot, and he goes from being a fat little ig to having some baggy skin on his side when he sits in certain positions. The first time it happened, I felt terrible, because he looked like he was starved. But now that I know it's mostly due to the season changing and breeding season coming on, I just watch him carefully. I make sure he's drinking water, and also that the lack of appetite doesn't last more than a few days.
I don't know a lot about your setup... iguanas will be more interested in their food if the cage is brightly lit, and they get enough UV and heat... also, you can feed your iguana a wide variety of greens such as watercress, spring onion, kale, collards, chard, mustard and turnip greens, frisee, and dandelion. If you have a Giant supermarket where you are, they have a produce guide book in the vegetable aisle, and I use it all the time to figure out which leaves are what.
Is he just not eating a lot? Or is there other stuff going on as well? How old is he? What's your setup like?