My 3 Year Old Was Diagnosed With Acute Idiopathic Thrombocytopenic

My 3 year old was diagnosed with acute idiopathic thrombocytopenic purpura. He has leg cramps and easy bruising and all test results(which are only labs) are pointing to ITP. I want to know if there is a more definitive diagnostic test? I know that a bone marrow biopsy is possible but they doctors wont do it until the ITP turns chronic (at one year after diagnosis) can you tell me if there are other tests that can be done that will spead the diagnostic process forward? For ex: US of Spleen, CT of the abdoman/pelvis, genetic testing, maybe even a spinal tap? I just am not sure they are diagnosing him properly, I am afraid he has a cancer that we are missing because they choose to rest their diagnosis on his lab results.