If you suspect it's the food, one thing you can always do is "reset" back to just cooked chicken. That is obviously very easy to digest. And if he still has problems with that, then it's definitely time to take him to the vet. Note that doing this is only to help nail down the problem, you can't feed him just chicken indefinitely, because it's not a complete food.
Iams is crap btw, one of the very lowest quality brands that can be purchased. You'd have to try extremely hard to find anything worse. This is what you are feeding your cat:
Ingredients: Chicken By-Product Meal, Corn Meal, Chicken, Brewers Rice, Corn Grits, Dried Beet Pulp, Chicken Fat
Try for something that has much more meat, no by-products, and much less filler. And wet food is going to be better for him than dry food as a general rule.
http://www.littlebigcat.com/nutrition/s ... -pet-food/