My 15 yr old cat is drinking alot of water but not eating. Help?

Ok so ive noticed my cat has lost a rapid amount of weight and is refusing to eat. But he drinks alot of water and urinates alot. His behavior is also normal. Like hes not acting sick or like anythings wrong. He also has a history of bladder stones and is on a prescription diet from the vet. Its royal canin urinary SO 33. Any suggestions as what could be wrong with him?