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I am sorry to hear you are worried about Clarence. I am a veterinarian and will do my best to help.
The most common reason for a cat that has been using the litter box to suddenly start urinating outside the box is a bladder infection. The only way to tell if he has an infection is by doing a urinalysis, and the treatment would be antibiotics.
Is Clarence neutered? If not, he may be starting to mark his territory by spraying his urine. If he is not neutered, then having him neutered should stop this problem.
Where did he go outside the box, on the floor or a rug, or did he spray urine on a vertical surface like a wall or furniture?