Open mouth lock jaw? If its to the point where he cant eat, he probably wont make it. You may have to make a hard decision. Let him live out the rest of his days slowly wasting away, or a kick mercy kill to put him out of his misery. I hate to say it, but if it doesnt resolve on its own, or if you cant assit him in closeing his mouth without causing any further injury or distress, you may need to go the hard road on this one.
Best of luck to you.