A 68-year-old man with Parkinson's disease presents with severe orthostatic hypotension (BP drops from 160/95 mmHg supine to 90/50 mmHg standing). He experiences syncope on standing and has been unable to walk safely. His medications include levodopa and carbidopa. Autonomic testing shows impaired baroreceptor reflex and reduced norepinephrine release. What is the most appropriate immediate next step in management?