Promaxol works very well for pain. Plus it is not harmful to the liver and kidneys. Being that your cat is quite geriatric I would suggest dividing the dose to start with so you can give it more frequently if need be.
Too much at one time might soften the stool until his intestinal flora gets used to it. He may not be affected at all but I prefer to start slowly. It is easy to give by just adding to food or water. If you add it to water please use water that is free of chlorine or fluoride. Another thing that would be helpful is a fish oil supplement. Be sure not to get anything with mint in it. Cats do not like mint flavor at all.
Please let us know how you progress or if you have any further questions.<br /><br />