Sounds to me like it might be mucus coming out of his anus. The intestines are lined with a mucus membrane and normally produce a certain amount of to help with the passage of stool. Excess mucus can be produced if the intesines are irritated or inflammed. What does your cat normally eat? Any chance he ate something he shouldn't have (like a plant)?
Some more info for you (although it pertains to humans, the concept is similar for cats): http://ibdcrohns.about.com/od/relate...mucusstool.htm