Is the cat high strung? IBS in animals can be a sign of "post traumatic stress", for lack of a better word. If he is, some sort of behavioral relaxor might be in order, although I'm loath to suggest that type of medication for an animal.
Try giving him cooked white rice. If he'll eat it, it's excellent for the digestive tract. Othetrwise, thetre's something called a ":pet patch" which is excellent for pets with digestive problems.
My parents took in a stray cat a few years ago that was finally diagnosed with IBS after MONTHS of diarrehea. The meds from the vet did not work so a friend who works at a health food store recommended Black Walnut liquid herbal extract. You can also find it on the web - Fresh and Natural website - 3 drops per 10 pounds. They do not put it in the water as directed but give it to the cat by dropper. It has a large alcohol content, but worked after a few months. They saw some relief after a few weeks but now the problem has been gone for months. Hope this helps!