Has anything at all changed about his litter box (if he has one)? We switched litterboxes to one with a lid and an opening to go through. I kept finding the rug in the bathroom being used shortly after that. I decided that maybe one of the cats didn't remember that we had gotten a new box or didn't recognize it. I was going to take each cat and put it in the box to remind them. That's when I found out the problem. One of the cats would have shredded my arms before she would go through that opening. We took the lid off, and problem solved.
Location and even change in the litter might cause a cat to find somewhere else to go.
Our previous indoor/outdoor cat didn't use a litter box. He just went when he was outside. We started having accidents with him on the bathroom rug. I don't know why all my cats like to go on the bathroom rug when they're having problems!! We first thought it was the dog having accidents until we actually caught the cat at it. The cat was getting older, and he just couldn't hold it as long as he used to. We had to be sure he wasn't stuck in the house all day or had access to the doggy door after that, and we didn't have any more trouble. (We also jokingly considered that he might have done it on purpose because we were blaming the dog. We figured he just sat back and laughed at the trouble he caused the dog, and the poor dog was thinking he not only had to keep himself from having accidents, but the cat was his responsibility now, too!!)
I don't know if your cat is getting up in age or unable to hold it as long either. Urinary tract infections and kidney problems are common in cats. Maybe it's time for a check-up at the vet, or taking a urine sample in.