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There are many different things that could be causing his problem. Since he has been normal for the last several years, it sounds likely that this is a medical problem rather than a behavioral issue.
I would consider a urinary tract infection, urinary stones, Cushings disease, or diabetes on the top of the list. I recommend having him seen by your regular vet. I would start with a physical exam and a urine sample. Depending on the results, we might need to proceed with more tests including bloodwork and xrays. At that point, your vet should have a diagnosis and be able to begin a treatment plan.
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