Two days ago you indicated that he wasn't eating... is he eating now? If he is, is he eating enough? Critters can get a condition ... ummm... in cats it's called hepatic lipidosis, in people it's called non alcoholic steatohepatitis I think... anyhow, when the body thinks that it's starving, it releases it's fat stores and they can overwhelm the liver. It's commonly referred to as 'fatty liver'. That, too can show signs like the sulpher colored poop. That color just indicates a liver problem, not what the problem is. The bottom line in that case is that it's critical to get food in him. Offer treats like grapes, catfood, whatever. You need to figure out if it's blackhead (contagious) or stress related. In either case, if his liver is having problems, his condition is critical now.