Seems like they think he has an autoimmune process consuming his platelets. Have they done any further testing to check for autoimmune disorders? The original diagnosis with the auto antibody was quite a long time ago. His temporary response to steroids could go with autoimmune disorder. Might be worthwhile consulting with a rheumatologist and/or hematologist to see what could be the cause of the low platelets.
For 7 years I had way too many platelets because my bone marrow was producing way too many of them. I was put on a low-level chemotherapy to keep the platelets under control for many years. Eventually my bone marrow stop producing platelets and I was on a medication called Vonjo temporarily which gave my platelets a boost. Ultimately I ended up with a stem cell transplant. Long journey (day 200 post-transplant) and having Specialists involved is a godsend. Never hurts to get a second opinion!
Lynn S.