A 25-year-old man is admitted with diabetic ketoacidosis (DKA). His initial labs show: pH 7.15, bicarbonate 10 mmol/L, blood glucose 28 mmol/L, ketones 3+. He is started on intravenous fluids (0.9% sodium chloride) and a fixed-rate intravenous insulin infusion (FRIII). After 4 hours, his blood glucose has fallen to 14 mmol/L, pH is 7.25, and bicarbonate is 15 mmol/L. What is the most appropriate next step regarding fluid management?