No one here bothered to actually discuss what's going on.
This is a 1-month old newborn.
He vomits after feedings, 5-day history.
At 1 month old, with vomiting after feedings, it is most likely a case of PYLORIC STENOSIS due to hypertrophy of the pyloric sphincter (usually occurs 6 weeks later).
Due to him vomiting stomach contents, he will LOSE Cl-, K+, and Na+.
He will ALSO lose H+. This is why you go into the metabolic alkalosis with a resultant increase in the HCO3-.