Get another fecal test if the vet feels its needed, and i suggest treating your ig with probiotics. Benebac comes in powder and liquid form at pet stores.. either in reptile or bird section. Usually probiotics will make them a bit runny too for a bit.
Soak your iguana in a warm bath, the water will stimulate him to poop, hopefully this will reduce the mess - better in tub than house surfaces. Also keep in mind every surface your ig walks over may be contaminated with that parasite and you can have the same problem.