Hypertrophic pyloric stenosis practice

A 5-week-old boy is admitted after a fortnight of vomiting that now shoots across the change table after every feed. The vomit has never been green. He feeds hungrily, has lost 200 g since birth and has a sunken fontanelle. Ultrasound has confirmed hypertrophic pyloric stenosis and the surgical registrar has accepted him. Venous blood gas shows pH 7.52, bicarbonate 34 mmol/L, chloride 88 mmol/L (95 to 110) and potassium 3.0 mmol/L (3.5 to 5.5). Which one of the following should be done next?