I will try to answer your query by presenting the question over here:
55-year-old patient was admitted in the hospital with the complaint of loss of sensation on the left side of the body. A few days later, the patient appeared to be improving and there was an evidence of return of sensation to the left side of his body,
This first improvement of health is because of the first medicines, intravenous/injectable/oral and thus the cells of the body start to function better. But if there is any further underlying cause, then this development lasts for a short time. This is evident in your next sentence:
but he suddenly started to complain of agonizing pain in his left arm and leg.
This implies the treatment that was started by the doctor is good. It has helped to understand the depth of the concern. If you might ask, "Was that all necessary?", I would say Yes. It was very much necessary otherwise further treatment would not had been in the correct direction.
The concern seems to be more with triggering of the nerves by the impulses sent by the brain and is leading to what you said further:
The pain would start spontaneously or be initiated even by the light touch of the bed sheet or by little exposure to cold.
This happens many times when the disease is not yet addressed in the treatment. The disease can be either from the nerve function which may indicate paresis/paralysis or seizure related concerns in the near future.
The pain failed to respond even to powerful analgesic drugs.
The pain failed to respond because even the most powerful analgesic if would not be the correct medicine, would never help in his pains.
The best way is to continue on the treatment with more focus on the nerves, to sooth on their triggering and hyper-responding to stimulus.
The options that you have now are:
1. Speak with your doctor and see if any different line of treatment may be involved.
2. Your doctor might not be thinking of further different treatments because of high cost involved in the same. Discuss about the same.
3. Your doctor might be having limited knowledge. Think of having a second opinion with a more experienced specialist.
4. Is the person being treated at a General Practitioner's hospital? Try and check at a specialty hospital. Government hospitals having specialist sections are often the best hospitals to check at.
You might want to check out for natural treatments like Acupuncture, Reflexology and Herbal medicines including Ayurveda and Homeopathy.