Iams is NOT the best out there just many of the other insanely expensive "natural" foods. Just like with human foods, READ THE LABEL. If grain or some type of "meal" is the first ingredient it's not "the best". The first ingredient should be some type of protein.
Dogs, like humans, CAN live on unhealthy food, but that doesn't mean it's best for them. Also, wet food has a far lower fat content that dry food. I was surprised when I learned this., but it's true. Try feeding him a wet and dry combination. This is really important when they get older, the wet food helps keep the bowels working easily.
Also, for treats give them NATURAL human food nothing processed. My guy loves carrots, apples and sugar snap peas as much as he loves a good piece of chicken. Again, plain, no sauces or condiments.