Getting discouraged

Hello, I am looking for anyone who may have similar experiences. My son is 13yrs old. He was diagnosed with ITP last October when he was 12. He spent a couple of days at children's hospital where he did IVIG. His numbers went up and then back down. We then did a dose of prednisone of 120mg a day and his numbers shot back up and then fell again. He did put on about 25lbs while on the steroids. We then did a 6 week stint on Promactica and saw no improvement. Today he had his 4th Nplate injection and is back on a low dose of steroids and his count was at 16. His Dr is out of Children's Hospital in St. Louis. She has told us the only test they have not performed is a bone marrow biopsy. My mother has MDS (myelodysplastic syndrome) which is a type of blood cancer. I have asked about this and have been told his cells look ok under a microscope. We are just concerned and looking for any advice. Thank you....