What's wrong with my green iguana?

I have had him for about 9 months, he was doing fine then the last few weeks he has lost a lot of weight and isn't moving around much. A couple days ago I started noticing he has patches of skin that are like dark almost black looking. I have a UVA/UVB light that I leave on for about 8 hours a day, I feed him a moist iguana food and fresh collard greens, occasionally I give him zucchini and squash when I have it around. Nothing has changed the entire time I have had him so I do not know why he is going downhill so fast! Also, I give him more food than he can eat, every day his bowl still has leftovers, I throw the old food away and give him fresh. Please help me, I don't know what I'm doing wrong or what might help make him better. Thanks