I empathize with the trouble your father- in law is facing.
Wear and tear of the muscles is known to happen with advancing age. There seems to be a probability of Osteoarthritis
in him by most chances. He will need to take Calcium supplements
on a daily basis.
The outcome of spinal surgeries is not very good, better results are seen in up to 50% of cases.
exercises would be best for him and slowly he can regain back his physical activities.
The treatment here is rehabilitation
and symptomatic management, rather than medications and surgery.