Pred has a half life and when it goes, provided it has not shut his adrenals down, he should be fine in a few days.
What doctors cannot or do not grasp is that some people just are more sensitive to any amount of corticosteroids, and can get into issues and develop Cushing's or Addison's from even small doses. If he had a condition that was undiagnosed before, the steroids would just aggravate it. I have seen it before in friends. Can you find an endo to help you? BTW, there is nothing to help the pain. I had Cushing's and when you wean after surgery, the pain is horrid but no narcotic meds help. I had my adrenals removed to cure it and now have to take steroids to live.
My son has had numerous courses of pred for asthma in the past. Usually 30mg a day and because it was only for 5 days the routine practise is to just stop them. It is usually if someone is on steroids long term that you need to gradually reduce steroids.
However, now he has adrenal insufficiency (unknown whether it will turn out to be Addisons) he cannot tolerate less than his 20mg of hydrocortisone a day. Even a 2.5mg drop causes return of symptoms!