We have a problem with our 13 yr male indoor cat, he's neutered and long haired. We have had him since we found him as stray as a kitten. Lately he has not been using the litterbox. We have another cat a female spayed who's about 4 yrs but I'm pretty sure it's not her. We have 3 litterboxes that are kept pretty clean. I check them about every 2 -3 days. Does anyone have any ideas on how to stop this or why he's doing this?