I am surprised that your doctor didnt tell your father-in-law to come in for regular blood tests, as a major side effect of hydroxyurea
is infections as a result of the effect it has on the bone marrow
I am assuming he is now off the hydroxyurea, so as such his bone marrow should start working normally again and his white blood cells will increase that should help in his condition.
Also ask them why they are giving him prednisone
, as yes it is given for infections. But i am not clear on how it helps your father-in-law's condition.
Hope this answered your question.