He's never had anything but Iams. That's why I got paniced because the store owner made it sound as if I'm feeding him crap. Should I switch or keep him on Iams? I'm so confused.
Like I said, I don't think there's a lot of scientific support for the bias against "byproducts" and grains. Grain free food can be too rich for many dogs or otherwise not the right choice, and "holistic" foods are often no different from "traditional" ones. "Holistic" doesn't have a binding legal or industry definition. Neither does "premium."
Grain free does work for some dogs, and it's certainly possible for a dog to be sensitive to a particular ingredient, so eliminating it does help. Most dogs, though, digest corn and wheat just fine in appropriate amounts.
If somebody is putting down Iams as a food, ask them for the actual nutritional research behind their claims. If they have nothing, don't sweat it. If they have something, read it and see if it's really comprehensive. Make those decisions for yourself as you see fit, but don't let a store owner make you feel guilty for not spending more on food. He's hardly an unbiased source, right? Iams has been producing dog food for a very long time and has engaged in countless studies to make sure the dogs thrive on it.
My theory is that if the dog is thriving, there's little benefit in changing the food because of some theory about grains or wolf digestive systems. If you're stressing anyway, why not feed him low-fat, low-sugar people food when you cook it? Our dogs love getting leftover chicken or other "real" meat as treats and snacks here and there. Check the list of human foods that are toxic to dogs before you do that, but if you're careful about that small number of dangerous ingredients, it's a great thing to do.