A 6-year-old boy from rural Maharashtra presents with 3 weeks of fever, bone pain, and bruising. CBC shows WBC 120,000/μL with 85% blasts, Hb 5.8 g/dL, and platelets 20,000/μL. Bone marrow confirms B-cell acute lymphoblastic leukemia (B-ALL) with high-risk features (age <1 or >10 years, WBC >50,000/μL). He is hemodynamically stable with no CNS involvement on examination. What is the most appropriate immediate next step?