Treatment for fibromylgia is first relax your body and mind because the symptoms are more when you are in stress and anxiety. Next you can use cold pack and hot pack to pain full muscles alternately to relax the soft tissue. You can also take physio therapy treatment for some more relaxing and healing effect on soft tissues. Here physio therapist will also suggest for few more stretching and muscle tonning exercises do follow that gradually. Also take suggestion on postural correction and follow it. There are couple of medications can be suggested but since he is already on cancer treatment I suggest that should be suggested by your oncologist only. And it may possible that he has pain because of prolong treatment of cancer with more stress on body and no physical exercises. and it may not be fibromylgia. So you can also discuss about this in detail with your oncologist to have further guidance. I am sure he will be fine with muscular pain if you follow the above steps. Take care.