Please feed wet food, not dry. Dry is dehydrating for a healthy cat, even if they drink a ton of water, as well as to high in carbs and low in animal protein. http://catinfo.org/ explains why cats are much better off eating wet food instead of dry.
If Eddie was on antibiotics you should really give him some probiotics. I use Holistic Select Transition Solution (store locator: http://www.holisticselect.com/store-locator.aspx ) The stuff that the vet would sell you would be FortiFlora but it has horrid ingrediants. The Holistic Select, you just need to sprinkle it on his wet food like it was alt and pepper, you don’t need much.
He could not be able to tolerate some of the ingrediants in his food. Try a grain free wet food which I would think could help. Wellness, Nature’s Variety Instinct, By Nature, and Before Grain all have grain free choices. The NV Instinct has novel protiens as well which could help.
Another option, the best one IMO, is to feed either a homemade or premade raw diet. I know a lot of people who use Nature’s Variety premade but there are other choices out there as well. It is better to not get the dehydrated raw but instead get the frozen raw.
Home prepared raw is what I feed most of my cats and kittens. It is best to either feed prey model raw or whole prey. Prey model is “make your own mouse”, http://rawfedcats.org/nature.htm is a good site to read to explain the basics.
Whole prey is buying whole, frozen mice, rats, quail, chicks, rabbits, etc and feeding them. The nice thing about whole prey is that it is balanced within itself. You can buy the whole prey off of the internet and have it shipped, frozen, to your house. All you need to do is thaw and serve 🙂