Please see a vet. It is most likely a URI, a urinary tract infection, especially if the pee is rather strong and stale smelling. Mention the drinking lots to the vet; it is a symptom of URI, but could be tied to other issues with the urinary tract, too.
In case of a URI ask for some metacam (painkiller and anti-inflammatory) for the first 2-3 days to help ease the discomfort and the acute symptoms if necessary. The best antibiotic for this job is septrin. Drinking is a good sign, the more he flushes his bladder, the better. You can help with mildly diuretic foods like cucumber and dandelion during the acute phase, but please don't overfeed; he may get diarrhea otherwise.
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