Talk to your father's medical oncologist. This is the doctor who plans, administers, and follows-up with the chemotherapy. There are some side effects of the disease and some from the treatment. It may be possible to adjust the medication dose to help with the disabling symptoms that can occur as a result of the treatment.
Many cancer patients experience a loss of function because of the pain they are having. Others find that their fatigue level and other symptoms combined with the pain keep them from completing tasks that used to be automatic.
There are some tests that can be taken by cancer patients to help identify just how much the pain is interfering with function. This guides the physician in knowing when to treat medically for pain relief or control.
One of those tests is a self-report survey called the brief pain inventory (BPI). This test measures pain interference with physical function such as general activity, walking ability, and normal work. It also measures the effect of pain on mood, relations with others, enjoyment of life, and sleep.
A social worker or psychologist can help you find the best test tools to use right now. Your oncologist will advise you what to expect along the course of cancer treatment. It may just be a matter of time before some of these functions are restored without actually doing something directly about them.