He should be very high on the list to be approved based on his history. All he should need to do is see his liver specialist, get a prescription and go from there.
I can't imagine any doctor who would not agree he should be treated although depending on his genotype he may need a different medicine that Harvoni but any doctor should agree to prescribe him the medicine appropriate for his genotype of hep c
Has your brother been evaluated for a transplant? If so, his team should be able to treat him with the proper medication. It is important for him to be followed by a hepatologist who understands how liver cancer behaves. Medicare should not be an issue.
My understanding is Medicare pays for Harvoni if you're sick enough. He certainly sounds sick enough so that shouldn't be an issue. The link below shows gastroenterologist/hepatologist who treat HepC in Arkansas. good luck!!