Well...Iams is one of those cat foods that people think is superior because it's more expensive, when really it's pretty lousy. There's a lot of by-product meal and corn products in Iams dry food. I'm probably preaching to the choir here, but a cat's diet should be made of almost nothing but meat (and bone, if they're fed homemade food). Cats DO ingest grains, and fruits/veggies in the wild....but they get those from the stomachs of the prey they catch, so it's in small amounts. And they do eat grasses/greens on occasion. But the bulk of their diets should not be corn or rice, and "chicken by-product meal" can be made out of beaks and feathers, and have no nutritional value. It's just cheap filler.
And yes, cats who are fed sub-par food will overeat, because their bodies are craving nutrition and it makes them keep returning to the food bowl in a never-ending quest for vitamins. So many cats (and dogs) who are fed junk food become fat.
If you can get your boyfriend to read some labels and pick out a commercial cat food that is made from meat and grain free, that would be the best thing. There is also a supplement called Missing Link that my cats like....you can find it in most pet stores, IME. You just sprinkle it on the cat's regular food.