Last year a friend who had a dog stayed with us for a few months. The dog sometimes stayed in the bathroom where the litter box is. After my friend and her dog left, my 10 yr. old neutered male cat stopped using the litter box. He will pea and poop on a towel in front of the covered litter box that he used for years but will not use the box. What can I do? Oh, he shared the box with our 6 yr. old female cat for years. She still uses it with no problem.