You may also suggest to your son that he check into the possibility of seeing a rheumatologist - many of them treat fibro patients and, in my experience, are more knowledgeable and "patient friendly" about it.
The restless leg symptoms that you describe are quite common with fibro/cfs. There are some very good medications that can help control it. It could be possible, too, that a prescription sleep aid would not only help the restless legs, but also help your son to get a better night's sleep - something that is very important with fibro. Many, many fibro sufferers have a horrible time sleeping (I know I sure do!) and need a sleep aid to help them. Of course, NEVER add or change any medication - prescription or over the counter - without first discussing it with your doctor.
I hope your son can find some relief soon. He's very lucky to have a mom who is so concerned and involved in his care :)
What you described of his legs jerking is a symptom that I've seen others with Fibro mention. He may have Restless Leg Syndrome and for that there are some wonderful prescriptions on the market.
With FM/CFS the pain seems to target anywhere and everywhere. There really isn't any reasoning as to why he may have pain in one location versus another... so it could just be that the pain is settling in his neck. I understand that this must be horrible for him and for you to see him suffer, but if he can find a Neurologist or a General Practioner that supports modern ideas about FM/CFS he should be able to get some very good prescriptions to control his pain and other symptoms.