solution unbearable pain after recurrent oral cancer

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My Dad was diagnosed with oral cancer in February. He is 43. He underwent a surgery in which his last 4 teeth and a part of the mandible(left side) was removed. A bone from his leg was put in the region from where the mandible was cut. After the biopsy 6 of his lymph nodes were infected from the 22 that were removed during the surgery. Just two weeks after the surgery he could eat drink and carry out all the normal routine stuff. After 45 days he took radiotherapy(tomotherapy) as suggested by the doctor. The PET Scan before radiotherapy had no problem. radiation therapy was over in 45 days(30 sessions) and all the side effects of the treatment were clearly visible. He eventually recovered from it. His radiotherapy was over sometime in may. In the month of July, his neck and shoulder started to pain. The intensity of the pain kept increasing, there came a time when he couldn't get out of bed. He took some physiotherapy as suggested by the surgeon but it was all in vain. In August the pain became unbearable. He got another PET Scan done which showed cancer cells in the surgical bed.currently he is undergoing targeted therapy. He has completed 2 of the 6 sessions. The pain was a little less during the last month but now his neck and shoulder is paining again with the same unbearable magnitude. his weight was 80kg before the surgery, now it is 54. Last week the doctors put a feeding tube in his stomach. He is on a dose of 3 morphines a day. What is the cause of this pain and what should be done in order to get relief?