long term side effects of radiation therapy

Can you help please, my father in law had cancer of the oesophogus (spelt wrongly I know) 18 months ago it was treated successfully with surgery and chemo, last October we discovered the cancer had come back and was in his brain, the surgeon operated in November 2009 and successfully removed the cancer, as a precaution father in law then had 2 weeks of daily radiation therapy on his head "to mop up any cancer cells that might have been missed" . Initially father in law seemed to be recovering well and although tired he had regained the weight he had lost since the first op, however since just before Christmas he has lost a lot of weight, he is very tired and becoming weaker by the day it seems, some days he is ok but today I had to go and help mother in law to get him out of bed and down the stairs as he was so weak, he is very shaky and his memory is becoming a problem, he has also started to lose all his hair again.  Is this normal after radium therapy to the head? Mother in law has been to the Doctor and they don't seem to say much, if it is normal and he will recover in time fair enough but should I be pushing for him to be referred back to the hospital it is just so worrying to see him almost fading away before our very eyes. By the way he has had a MRI and appears to be cancer free.  Any thoughts or advice would be great