A 42-year-old male construction worker from Uttar Pradesh presents with a 4-week history of dermatitis on sun-exposed areas (face, neck, hands), chronic diarrhea, and mood disturbance. On examination, he has a well-demarcated erythematous rash with scaling and lichenification in a glove-and-stocking distribution. Serum albumin is 2.8 g/dL. He reports a diet consisting mainly of polished rice and legumes. What is the most appropriate immediate management?