My sister in law had a similar problem with her cat. He had bloody diarrhea, was vomiting, being lethargic, refused to eat, lost weight to a point where he was only about the size of a ferret.
3 different allopathic vets found nothing and told her there's nothing wrong with him at all. Yeah right, obviously
something has to be wrong if an animal is unhealthy like that!
She finally found a holistic vet who specializes in cats, and she diagnosed Talis with inflammatory bowel disease. He went on a diet of canned food only (with certain restrictions) and was given two different fiber supplements and probiotics and now he's doing fine. Of course this disease will never go away and occasionally he has another outbreak, but not as bad anymore as he used to experience.
I highly recommend you find a different vet, check out
http://www.holisticvetlist.com and see if you can find one in your area.
Meanwhile, throw out the Iams and fast your dog for 24 hours.
Do not feed any solid food so the digestive tract gets a rest. Small amounts of nutrical or honey are okay, and give plenty of water, or even better, pedialyte.
The
only thing I'd recommend as a supplement (during fasting and after) is slippery elm. One heaping teaspoon of finely powdered bark mixed into enough cold water to make a paste and then quickly stirred into a pint of boiling water, let cool and give a teaspoon or two every couple hours.
After the fasting day, feed a mixture of 2 parts white rice (short or medium grain, cooked to death, 1 cup rice to 3-4 cups of water, until you can squish a grain of rice to pulp between your fingers easily) to 1 part cooked skinless chicken or turkey breast with the fat trimmed off. It would be a good idea to run it all through a blender or food processor until it has the consistency of baby food. Short-term you don't have to supplement anything, but if you feed this for some time, you should add 50 mg of calcium supplement per ounce of cooked chicken breast. One pound of this food supplies approximately 625 kcal, compare that to the amount of calories in the daily serving of Iams you were feeding to determine how much to feed.
Do not substitute brown rice for the white, you want something that is easily digested and low-residue, in order to irritate the intestines as little as possible.