A 6-year-old boy from Delhi presents with a 2-week history of fever, cough, and progressive respiratory distress. His father was treated for pulmonary TB 4 months ago. On examination, the child is febrile (38.5°C), tachypneic, and has bilateral crackles on auscultation. Chest X-ray shows innumerable 1–2 mm nodules diffusely distributed throughout both lung fields, creating a 'millet seed' appearance. Mantoux test shows 15 mm induration. The child is otherwise immunocompetent. What is the most likely diagnosis and the immediate management priority?