Lori,
I am also one who has followed your posts for several years. I'm very sorry to hear that your son's scoliosis has worsened. I don't know much about the surgery to correct it, but words like full body cast sound terrifying.
I quickly googled "surgery for scoliosis for teenagers" and found lots of support groups. Maybe if your son meets other teens online that are facing surgery or those who have already had it, then he won't imagine the worse case scenario and perhaps see some positive cases.
As all of us have found here, sometimes you just need to talk to someone who is going through the same experience.
Let us know how his is doing. We will keep him in our thoughts.