Conversion Disorders in children and teens can be difficult to discern. It is not as if the child is pretending there is pain - there actually is pain, but there is no medical reason to explain the pain. The key is to be sure that all medical explanations have been ruled out and that all specialists are concurring in the diagnosis of Conversion Disorder. If the diagnosis is accurate, your son is in the type of facility he requires. But be sure to obtain second (and even third) opinions.
Perhaps (while waiting for the response) you could fill in a little more info such as your family situation...divorced? happily married? other children in the family, any issues other than this, such as bedwetting, aggression, unusually shy child, etc? How do you react when he is crying out in pain?
It may help the therapist formulate a response.
You have my thoughts and prayers--what an ordeal to go through. I hope the therapist comes up with some ideas for you.