As I can understand from you post your father is not able to open his mouth and he has pain while doing so.
There can be many causes for this problem- an infection in a tooth, muscular spasm, pericoronitis
(infection of the third molar), problem related to temporomandibular joint. As you said you doctor said "it could be due to a nerve". This is a very generalized statement and I would advise you to take you father to a Dentist
who is qualified to understand you father's problem. The possibility of a bad tooth can not be ruled out until a proper clinical examination is done.
In case you have any other queries please feel free to contact me at +91-7838386006.
Dr. Manish Juneja BDS, MDS, DNB, Cert. Oral Implantology