Did you find out what this was? I had a similar problem when my cat was a kitten and he had worms. He was very smelly, runny stool, and his anal area would bleed after going to the bathroom.
Thanks for this help - I'll take him to the vet on Monday to be checked out - and have already changed his food (which seems to be helping). Thanks again - I'll let you know how the vet goes :)
Did your vet give you any info at all on what this could be? Where all his tests normal? The simplest answer is diet. If there's blood...it could be a minor recurring infection. Otherwise, this kind of thing is usually a sign of digestive issues. You may want to try a "bland kitty diet", something high in carb/protein with no additives or other chemicals.
You might try changing his food. My girl kitty could clear out a room when she was little- she was sooo stinky. I changed her food and discovered it was one specific variety of beef flavored can food. She has not been near as stinky since, but every once in a while something gets her going.
I would take him to the vet to be checked out if I were you to see what's causing the problem.