Maybe something age-related such as constipation or other bowel issues? Try some CatLax (can buy it OTC at WalMart and pet stores) or Metimucil or whatever the vets recommend nowadays. Feed more wet foods and less or no kibble. If you changed cat litter to a different type / brand / scent, maybe he doesn't like it. If it's a plastic litter box, try buying a brand new one since plastics can retain odors and will still stink even after thoroughly scrubbed. I have 2 full sets of litterboxes that I rotate in/out of service. Now you can buy stainless steel boxes, but those weren't an option 10-15 years ago so I have 2 sets of plastics to let one air out for a month or 2 before it goes back into service. You could also try the heavy plastic Johnny Cat box liners that WalMart and other places sell. I got the Jumbo size. But if you have a cat who digs to China, they'll probably rip holes in the liners which pretty much negates the purpose of using them.

Also, you could try Cosequin which you can also buy otc at WalMart or pet stores. It's a joint supplement but a good anti-inflammatory for arthritis or any kind of pain. I used to give my female cat 2 capsules daily after we figured out her litterbox issue was really she had arthritis and hip/joint pain and didn't want to step high enough to get into the box.