Once in a while, Miss Bo will have "essence of cat box" hanging around her, so I check her behind, and sometimes I will find and clean off bits of solid waste that have clung to her fur. This usually clears up the disagreeable odor.
Does Herman eat people food? How about dog food? A friend's cat had to be restricted from eating the dog's food because it gave the cat a really bad case of gas all of the time. If Herman goes outdoors, perhaps he is eating the food that well-meaning neighbors leave out, or that the occasional mouse gives him the stinks. Just guessing, cats are secretive by nature and it's hard to know for sure what they are eating unless they are strictly monitored.