karkinos posted:

How old is your cat? Eleven is considered old enough for cats to develop senile dementia, although they can occasionally develop it earlier, and one of the most irritating side-effects is improper defecation. Even if he is not old enough to have developed dementia, other neurological conditions can impact their ability to determine where they are.

Frankly, if all the precautions you've taken haven't worked already, he is either suffering from a neurological condition or a little disease I like to call "being a huge dick", because the only other explanation is unresolved aggression. Typically, when cats elect to poo poo on top of the litter and not cover it, it is a direct confrontational tactic used to intimidate other cats (or, in some cases, people). Defecating outside the litter box without a present condition is exceptionally aggressive behavior and is usually only parceled out in times of extreme stress. Presuming this is the explanation, changing the type of litter and adding another cat might have exacerbated the situation.

In either of these cases, typically the only solution is to pay more attention to him, which may not be an attractive prospect based on the fact that he shits in your house like a mental patient that is trying to have his recreation privileges revoked. Dementia is often cooled by changing their environment (taking them for walks, changing furniture), and aggression is soothed by the exact opposite (timed meals, scheduled grooming).

Check the symptoms for dementia before getting him evaluated for it, since most of the "routine" tests run the gamut of x-rays and blood tests, which depending on your area, are wildly expensive.