Well, you're going to find that many BPOers, and I guess I must disclose that I'm included, will tell you that anything made by Iams is really um, inferior food.
Here's the thing, though. It might actually firm up his poops. They put firming agents in there so that happens. That doesn't mean its a good food or that he's getting good nutrition.
Just my opinion, but I would not have him on Iams. There's just not a lot of high quality ingredients in there. I would try him on a high quality food and go from there. What was he on before the vet put him on the Iams?
FYI, many vets are not trained in nutrition. They're not your best source of info in this area.
Here are the ingredients, maybe Holly will come along and translate them for you:
Potato, Herring Meal (source of fish oil), Catfish, Animal Fat (preserved with mixed Tocopherols, a source of Vitamin E, and Citric Acid), Dried Beet Pulp (sugar removed), Fish Digest, Calcium Carbonate, Dicalcium Phosphate, Sodium Hexametaphosph ate, Choline Chloride, Potassium Chloride, DL-Methionine, Ferrous Sulfate, Vitamin E Supplement, Zinc Oxide, Ascorbic Acid, Ethoxyquin (a preservative), Manganese Sulfate, Copper Sulfate, Manganous Oxide, Vitamin A Acetate, Calcium Pantothenate, Biotin, Rosemary Extract, Thiamine Mononitrate (source of vitamin B1), Vitamin B12 Supplement, Niacin, Riboflavin Supplement (source of Vitamin B2), Inositol, Pyridoxine Hydrochloride (source of Vitamin B6), Vitamin D3 Supplement, Potassium Iodide, Folic Acid, Cobalt Carbonate