What are his doctors doing for him at this point? Have they identified the cause of his Cushing's?
They are looking for an ACTH-secreting tumor. Removal of any such tumor is the mainstay of treatment and may be curative. This needs to be as soon as possible in order for him to get better as soon as possible. If you cannot get these studies scheduled in a timely manner, you could try going in to urgent care or the ER and requesting it be done on an emergency basis, given his clnical deterioration. I would start with the MRI of the brain, then do the abdominal study (CT or MRI) if the head is clear.