A 28-year-old male farmer from rural Maharashtra presents to the emergency department with a 10-day history of fever, anxiety, and progressive neurological symptoms. He reports a dog bite on his left hand 6 weeks ago, which he ignored. On examination, he is agitated, has excessive salivation, and displays hydrophobia when offered water. CSF analysis shows lymphocytic pleocytosis with normal glucose and protein. Brain MRI reveals T2 hyperintensity in the brainstem and hippocampus. What is the most appropriate next step in management?