Teeth grinding could indicate a problem anywhere
in the digestive system, not just the mouth. If he has mouth pain, or arthritis
in the TMJ joint your vet could try giving an analgesic called Metacam, or a short course of steroids called Prednisolone. And, place your cat on a higher quality all canned diet.
Losing 14 pounds in two weeks may indicate a much more serious disease process occurring in your cat, I advise a complete work up if he has not had one already prior to the dentistry.