A 23-year-old man with type 1 diabetes presents with nausea, vomiting, abdominal pain, and Kussmaul respirations. Glucose is 26 mmol/L, venous pH 7.12, serum ketones positive, K⁺ 5.3 mmol/L. According to standard DKA protocols, what is the most appropriate initial management step after airway and breathing assessment?