Has he been seen by the same vet all this time? Maybe you need to get a second opinion, before your cat loses any more weight. Was a blood test done? Did they actually identify the bacteria?
Metro can also cause diarrhea. If it was going to work, you probably would have seen an improvement by now. Ditto the probiotics.
You didn't mention blood or mucous. Are those present too? I'm not saying it's not colitis, although usually if the animal was that sick, you would see more evidence. Is he running a fever? If he is truly dehydrated, it's even more important that you get him to a vet.
Have you tried some invalid food, such as boiled chicken and rice? I'd start with just the chicken-flavoured rice, cooked very soft. Let him eat small quantities as frequently as he wants.
You might also want to try slippery elm, a herbal powder that can be purchased at the health food store (if it's in capsules, make sure there are no additives). Mix a tablespoon with a little hot water and let it cool. It should thicken. Administer with a syringe. It coats the digestive system and also has curative powers. Might make him feel better.
Last edited by badger; January 27th, 2009 at 11:24 PM.