Thanks for your question. Prednisone is a steroid and will not cause him to stop drinking; in fact it should have the exact opposite effect and cause increased thirst.
If he is disoriented, he may no longer recognize the need to eat or drink. He may also be in pain which is causing him to stop taking in food and water.
I have always had good luck putting a little chicken or beef stock in some luke-warm water to encourage drinking. However, I have to say that if he is refusing to eat and drink, then this is a sign that his condition is worsening. At some point, you will need to asess his quality of life and decide what is best for him.
You've already gotten some good avice but let me suggest a way to get fluids in if he isn't drinking - you can take a sports bottle or a basting syringe and slowly trickle water in between cheek and gums. You tilt the mouth up a bit to make sure the fluid flows down to the throat so he has to swallow. Make sure to give him time to swallow with breaks in the flow.
Hope this helps you!