I am also very happy that you've finally found a doctor who is willing to help you and who cares about you. He is correct, there are many lupus patients who have had negative lab results. You results could be negative this month, and show up positive next month and be back to negative again the following month. It has happened and it is not that uncommon!
Prednisone is a corticosteroid. It is most often are used for their its potent anti-inflammatory effects, particularly in those conditions in which the immune system plays an important role (such as Lupus). Prednisone is also an immunosuppressant and it affects virtually all of the immune system. These two reasons are why it is often used in autoimmune diseases, inflammatory diseases (such as asthma and Crohn's Disease), various kidney diseases including nephrotic syndrome, and to prevent and treat rejection in organ transplantation.
Corticosteroids typically do not produce immediate effects and must be used for several days before any effects are seen at all. However, it may take much longer before your condition responds to the prednisone.
Peace and Blessings
Look For The Good and Praise It!