A 4-week-old boy is brought to your OPD by his mother because he has had increasing amounts of vomiting over the past week. Initially it staed as "spitting up" after a few meals a day, but now the baby is having projectile vomiting after every meal. She says the vomitus is non-bloody and non- bilious and the baby appears hungry after he vomits. This is her first child and she is not sure if this is normal. Physical examination is unremarkable. Laboratory studies show: Sodium: 140 mEq/L Potassium: 3.0 mEq/L Chloride: 87 mEq/L Bicarbonate: 30 mEq/L At this time the most appropriate next step is to