I was just reading about Myositis and found an interesting quote about it:
"Most experts in the natural medicine field believe that many of these autoantibodies are produced by the body in response to an infection or allergen. When the offending infection(s) or allergen(s) are removed, the production of these destructive autoantibodies stops."
I know in one of your earlier posts you mentioned that the Dr. thought perhaps your son had picked up a virus. But if it 'could' be a response to an allergen or infection, it sounds like things could improve if the trigger was found. (if there is one to begin with).
I'm sure he's not up for 101 questions but maybe when he's feeling a little better in a day or two, he could try and determine if he's been exposed to any new things, foods, enviroments etc..
I hope that he will get better very soon. I know this must be so hard for you, especially since you are far away. I hope you get good news about it all in a day or two.