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
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!!
http://www.hepatitiscentral.com/hcv/drs/ar/
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.