You may be observing something doctors call response shift phenomena. It happens to many, many patients before and after surgery (or other treatment). They have less pain and better function, but they rate their progress as worse than before treatment. They seem to see themselves as more disabled even though tests show they are stronger and more active.
The patient has adapted to the new level of ability and then his or her expectations change. It’s a bit like a moving goal post or using a shorter yardstick for measuring desired outcomes. Showing the benefit of treatment becomes a challenge because patients change their internal standards for how they view their pain or other symptoms. They may reassign importance of one symptom over another.
Your father’s example is a good one. Someone with back and leg pain from degenerative disc disease has surgery to remove the disc. After surgery, the leg pain is gone but the back pain is still there. Even though half the pain is gone, the patient rates pain-related disability as much worse than before surgery when there was back AND leg pain.
This patient has had more than just one type of response-shift phenomena. Besides changing internal standards of pain, now he or she has reprioritized the relative value of pain severity. In the absence of leg pain, suddenly the same level of back pain is much worse and more disabling.
It may take some time before things settle out and your father recognizes the improvement in his function and quality of life. At the same time, the doctors and therapists can now address the back pain and see if additional treatment can help. It is possible that gently pointing out the changes you have observed may help your father take a step back and re-evaluate his own situation. This may or may not help create a better appreciation for the results of treatment so far. After all, pain is pain and it has a way of getting our full attention!