I was hoping to hear back from you so we could discuss a little more information about Buddy, but I will send you my answer now, and if you have any additional questions or information you would like to provide, just click "Reply to Expert."
It's possible that he could have developed a urinary or intestinal obstruction, unless you have seen him use the litterbox normally for both urination and defecation. Urinary blockages in male cats are medical emergencies, so if you have see him making frequent trips to the litterbox, with either a few drops or nothing coming out, he needs to be seen and treated by a vet, immediately.
I hope he's eating and drinking as usual. If his appetite is not good or he's not eating at all, this can indicate a problem in the intestinal system and when a cat doesn't eat properly or at all for even a short period of time, this can lead to liver disease.
Due to his senior age, he may have some arthritis and it's difficult for him to get around because his joints are painful. Notice if he limps when he walks or if he walks more slowly than usual. Also, look for any weakness in his hind legs. Older cats are more prone to develop diabetes, which can lead to diabetic neuropathy causing weakness in the hind legs. Other symptoms are drinking more than usual and eating more or less than usual. Hyperthyroidism and kidney failure are other common medical issues in older cats.
Since this has been going on for 2 weeks, it would be advisable to have Buddy seen by his vet, as soon as possible, so a hands-on physical exam and lab work can be done. After that, you will get a diagnosis and treatment, if needed.
I hope all will be well with Buddy and please let me know how he's doing an what the vet tells you. Thanks!
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