Cat Always Drinks An Oddly Large Amount of Water?!

In Veterinary medicine it is called PU/PD (polyuria/polydipsia). This just basically means what you described, drinking a lot and urinating a lot. Does you kitty have any other symptoms such as vomiting or weight loss? You mention that he's been doing this for 5 years, which wouldn't concern me that much if he has been healthy otherwise. Some causes for PU/PD could be hyperthyroidism and kidney disease, both very common in older cats. I would take him to the vet to make sure everything is okay now that he's getting older. Just some routine blood work should be able to reveal anything suspicious. I would be happy to help if you have any other questions regarding PU/PD.