If it is a he, the poor thing probably has feline urinary tract disease which is an indirect result of the things we feed our cats for food. Most of it has ash and other fillers that cause problems in male cats, mine is near that age and has had it for a couple of years and is still going strong, the cat that is. As time passes however it will be harder and harder for him to make the box and he is getting to the point where other things will begin to take away his quality of life, it is time to get a kitten and have the old cat teach the new cat how to be your cat. Most of the times they will even play with each other, but they learn from observation so getting one now will also lessen the pain of the passing of the old cat. It is one of the problems when we that live 70+ years on average take on animals as house mates that only live 15 or so at best, there is bound to be some hurt and loss and rightfully so. The chances of it being an infection at that age are pretty small but it will not hurt to have him looked at by your vet. He can tell you about the disease and the progression rate and give you some ideas that may make him or her, more comfortable, good luck with your kitty.
I would think it was some kind of infection--get that cat to the vet especially since its 14