Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Moustache's lethargy and seeming pain when trying to open his mouth.
There are several disease conditions that can interfere with a dog's ability to open his mouth normally. It is important to determine whether he can open his mouth, but it is very painful to do so, or he has something which is making it difficult to open his mouth and his crying is related to anxiety because his mouth isn't opening normally.
My primary concerns in a dog that cannot, rather then doesn't want to open his mouth because it hurts are:
1) masticatory myositis or polymyositis (which are both autoimmune diseases where the body attacks its own muscles leading to inflammation and eventually scar tissue formation in the muscles). Diagnosis is by a muscle biopsy of affected areas, and/or an antibody test called a 2M antibody test for masticatory myositis.
Here is a link that discusses polymyositis in dogs: http://www.petmd.com/dog/conditions/musculoskeletal/c_dg_polymyositis_dermatomyositis#.UVSXkc7n_IU
2) tetanus (a bacterial infection that produces a toxin which interferes with nerve function. Diagnosis is based upon the classic symptoms and facial expression as well as a history of a puncture wound. Here is a link to an article about Tetanus in dogs: http://marvistavet.com/html/tetanus_in_pets.html
3) A less common cause would be certain types of protozoal parasite infections.
If he truly cannot, not will not, open his mouth possible causes could be a lodged foreign body, oral abscess or temperomandibular joint pain.
A jaw dislocation is possible but should be secondary to significant trauma. That would be diagnosable with radiographs of his jaw.
Sometimes we need to sedate these pups to determine whether it is too painful to open his mouth or he truly, physically cannot open his mouth. At that point we can look for a foreign body or abscess, take radiographs of his jaw and collect a blood sample or muscle biopsy to look for myositis.
If he has had a recent puncture wound and has the facial expression of a dog with Tetanus then treatment with antibiotics, such as Penicillin in the injectable form is recommended.
He may need a feeding tube placed to support him while he is responding to therapy and intravenous fluids to guard against dehydration.
Please let me know if you have any further questions.