We have adopted an older (13 years) Persian cat from family friends.
About 4 months ago the cat started losing weight and stopped eating dry food.
We took him to the vet who did full blood work for a variety of
disorders and diseases associated with older cats with these symptoms.
He weighed only 8 pounds at this time.  He took two weeks of
antibiotics and we started feeding him pate wet food mixed with water
and 'vita gravy'. He will lick the food until it is dry and will eat
three or four times a day. We took him back to the vet to get his
teeth cleaned and checked to make sure his teeth were not bothering
him. He had lost a pound down to 7 pounds. To the vet's surprise she
could not open his mouth when he was under anesthesia. There was
nothing wrong with his teeth. She explained that although arthritis
would sometimes cause cats to not open their mouths b/c of pain that
you should always be able to open them under anesthesia. X-rays were
inconclusive. The only thing she could come up with is that the bones
had grafted together from lack of movement b/c of pain from arthritis.
He has always been an indoor cat and has not suffered any injury as
far as we know. Now we are broke and still do not have an answer! What
could cause this? Tetanus? Bone fusing?

There is no way to make a diagnosis over the Internet. You may want to
ask your vet to investigate muscular causes rather than skeletal ones.
For instance, I believe there is a feline variant of this condition:

"Masticatory muscle myositis (MMM) is a common focal inflammatory
myopathy (myositis) in dogs selectively affecting the muscles of
mastication and sparing the limb muscles. Clinical signs may range
from acute swelling of the temporalis and masseter muscles, restricted
jaw movement, jaw pain, and exophthalmos to muscle atrophy with or
without restricted jaw movement. A classical clinical sign of MMM is
inability to open the jaws under anesthesia."

http://medicine.ucsd.edu/vet_neuromuscular/cases/1999/may99.html

Another (non-muscular) possibility might be temporomandibular joint ankylosis.

I don't know what could cause this, but if it were my cat, I'd get an
osteopath to look at it, just in case some manipulation could free
something off if it's stuck or jammed.  I would favour an osteopath
over a chiropractor, because their training is different and I feel
that some of them are better suited to working with animals.  Just my
opinion.

Jane

Please, get a second opinion from a good vet immediately!!