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I am sorry that you have been waiting for a response. I recently just logged onto the site and noted that your question hasn't yet been answered. I am a licensed veterinarian with over 25 years’ experience and would be happy work with you if you are still needing assistance.
It sounds as if he may have a temporomandibular joint (TMJ) injury or problem or somehow the joint is not working correctly. An xray will most likely be needed to confirm. There is really not much that you can do to help this until you know the cause, but in the meantime, about all you can do is to try to support his eating and drinking efforts. Offering soft food that is more easily lapped up instead of chewed will probably help some also.
It may offer some relief as well to try cool compresses to the affected side of his face. It may help to decrease inflammation as well as ease any muscle spasms that may be occurring because of it.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.