A 52-year-old man with a 10-year history of type 2 diabetes presents to the pain clinic with chronic neuropathic pain in both feet. He describes a burning sensation with numbness and tingling. Visual analog scale (VAS) score is 8/10. He has already tried gabapentin 1800 mg/day without adequate relief. On examination, he has diminished vibration sense and absent ankle reflexes bilaterally. What is the most appropriate next step in pharmacological management?