"I have a 7 year old overweight cat (who lives with me and 3 other normal weight cats) who has, what seems to be crohns or inflammatory bowel disease. He has been checked for everything but the vet says it's inflammation. He gets spasms when he poops and there is always blood in the stools which are watery. He's eats and plays and runs really well for a fat cat! What is going on and how can I help him?"
Inflammatory bowel disease is very hard to diagnose and to treat. Unfortunately it is a blanket term for a problem with a variety of different causes. Causes of inflammatory bowel disease range from: parasites, fungi, bacteria, food allergies, "leaky gut syndrome", and sometimes even cancer. You will need to talk to your vet about what you cat has been tested for. They have probably ruled out some of these causes and that might give you a better idea of what you are dealing with.
It may also be an idea to talk to your vet about a homemade diet as well. The vet will be able to give you the basic outline of what your cat needs to eat for full nutrition, like a taurine supplement. With this basic outline you can modify your cat's diet to rule out any food allergies or protein intolerances. Although it may seem like a lot more work than pouring kibble in a bowl, you can actually pre-make the food so you just need to serve it. All of your cat's can eat the same food as well. If a homemade diet is not feasible, look into a good natural food that has a clear list of ingredients so you can monitor if your cat is reacting to it.
Unfortunately there is no clear cut treatment for your cat without a specific diagnosis for the cause of the inflammatory bowel disease. I hope I have been able to provide at least a little more information.