The TMJ joint is the joint that hinges the Maxilla to the Mandible, and can suffer the same orthopedic problems as any other joint
in the body, such as strains, sprains, dislocations, and arthritis.
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.