That sounds similar to what happened to my Dad. (but could be different,
because I don't know if the brain controls muscle function)
Will you let us know if they find the cause?
I've been searching for quite a while, because we want to know if it's
something genetic perhaps in combination with one of the anesthetics used.
In those days, they threw out the empty containers, so (as a result)  they
could not be checked to make sure that he received what they thought he was
supposed to.
This did a lumbar tap and a myelogram and ruled out some things, including
disc protrusion or arachnoiditis.
They claimed it might have been due to nerve damage due to meningitis, in
the war years.
Now, my brother needs back surgery and he's too scared to have surgery.
What's your Dad's age? Mine was 50, in 1967 - it was not a brain tumor
surgery.

It started off post operative with "a paraparesia and sensory loss to the
level of approximately thoracic-10.  The extensors and flexors of the toes
and to some extent of the feet were functioning within a few hours but
there was marked weakness of the hip flexors and extensors.  Further in the
report that I have mentions "considerable weakness in his hamstrings and
gluteae".
If I recall correctly his bowel would plug up frequently (surgery for the
colon cancer later solved that) and hie had urinary incontinence.
He essentially had to learn locomotion again (how to propel his body using
a type of hip swivel to move one leg, then the other, to walk, often with
the use of a cane, sometimes a walker and sometimes had periods of only
being able to move around in a wheelchair. If you look up
relapsing/remitting MS, that's the sort of life he had thence on.
Sometimes I wonder if he did have MS. (there's no mention of MRI in the
report I have on it, I don't know if the technology wasn't available at the
time or was).  His mother (our grandmother) spent the last 40 years of her
life in a wheelchair and we never found out what the cause was or if they
told us, I forgot. She was mostly cheery but had no mobility, except upper
body.

He did physiotherapy and hydrotherapy for several years and had varying
degrees of success.
It helped strengthen his upper body muscles which helped him a lot, even in
wheelchair and/or other ways to get around. And he continued, even when
watching TV, to "work" his leg muscles.
I cannot emphasize enough how important this is, if he's still of a younger
age, so he can survive on his own and prevent being put into a nursing home
or ending up with pneumonia,  That's eventually what killed Dad, but he had
a life, for many years, (survived colon cancer, dealt with skin cancer),
due to his hard work, working the muscles.

I'm sharing this for things to ask about and/or if your Dad's doesn't
improve, how to help him to help himself to keep his muscles going.
If you find out a different cause, please post. (in case it's a similar
cause that we (family) need to get checked out. I may have upcoming surgery
and it's a worry (some).