Hi Joan,
18 is a pretty good age for a cat! Good work on taking care of him so well for so long. I would be suspicious that his arthritis issues are contributing to his lack of litterbox habits. If his hips or other joints are too sore, he may not want to climb into the box as often but will go in to urinate. He may need more medication than the Cosequin. Other things to consider are the height of the edge of the litterbox and the location (do you have one on each floor?). If not his arthritis issues, it could be a behavioral condition but this would involve ruling out medical issues first. A visit with your veterinarian is definitely warranted.
I hope this was helpful.
Dr. Ryan Llera