we are struggeling with our lovely cat for almost half a year now so I just try to give a brief history of what was going on in the past, what happened to explain the situation.
We received our cat 'Mango' over 3 years ago when he was 6. He was caponized, lived with another friend of us mostly alone, sometimes with other pets around. He is and was a home staying cat. Everything is great. He is a british short hair cat.
Last december in 2017 we received another cat, Mimmi. A female cat of age 2.5 years, also a home staying cat. the first days were like crazy but soon it all went smooth - of course they hunt themselves on a daily basisor she wants to play a little roughly with him, but really from what we can see not really mentionable. They can lay next to each other - no touching - but no problems on the couch or on the bed. No matter if we are at home or not. She is a european short hair cat.
After around 3 months we thought everything is great - they are not lonely - it looked really good for us. But out of heaven - Mango started to stop using the cat litter box. In fact what he started was peeing right in front of the litter box. We had two boxes at that time and it did not matter which one. We did not exchange the insides or anything else.
Just some weeks more he also started to poop right in front of it.
We bought an additional litter box and placed it at the other side of the flat. But nothing changed.
What we achieved is, that sometimes we take him to the litter box, put him inside, and it works for that moment. We also have the situation that in very very rare cases he uses the litter box by himself. But around 98% of the time he ignores it. After a while he also started using the bathtub or shower to pee or poop inside it (well at least its easier to clean..) but in my opinion the same situation.
The litter box is always clean - we clean it in the morning and evening. We exchange the insides like cat litter and clean it completely every 3-4 weeks like we did it before that situation.
We have no idea what we can do to help him or to identify a problem we cant see yet. The vet says he is healthy.