My Iguana is having tummy problems?

I have an iguana of about 4 feet, I've had him for about 3 years, he is on a healthy diet of greens and such, along with calcium supplements and heating arrangements, and he gets sunlight all day and he is free to walk around. Lately he hasn't been eating much like he used to, he only eats one leaf or two, and he is having diarrhea, it's very wet, jelly like, and it doesn't hold it's shape and it has a very strong (unpleasant) smell. what could be wrong with him?