Old cat poops in the sink, how to stop him?

I moved into a mobile home park, for the Soul propose of living next to my aunt. My uncle had just died and she needed some one to care fore her. Three years after we moved in she passed away. When she died she left behind a 14 year old cat that she had raised from birth.

Now, my aunt was a hoarder. She lived in this trailer with my uncle fore almost 40 years. She still had everything that she owned from then til her death. Her death was in 2012. When my uncle died, my aunt stopped cleaning up after the cat. He was allowed to poop and pee where ever he wanted, and most of the time it was in the bathtub and the kitchen sink.

When we ended up taking in the cat, he was fine at first. He was good about using the litter box and followed the rules wonderfully. Then we started bringing some of my aunts stuff into our trailer. we put a shower curtain up from her place in our bathroom. suddenly he started pooping and peeing in our bathtub and in our kitchen sink. i would clean it up and he would do it again. He even stopped using the litter box. i don't know what to do.

i just want to know, what is this behavior, and how can i get him to stop?