Re: low blood platelet
There are several causes for that. The one I have is known as immune thrombocytopenia (ITP). The body's immune system makes antibodies which turn against the platelets and destroy them. Mine haven't yet been low enough to put me in danger, but if it gets to that, there are treatments to boost them fast and high enough to alleviate the immediate danger of a major bleeding event.
Prednisone (or other steroids) and IVIg are the two first-line treatments for dangerously low counts. If those fail, there are others.
If your stepson has not seen a hematologist, he needs to. I hope the cause of his illness is identified soon. Best wishes to all of you.