That could be Mallory-Weiss tear a condition refering to bleeding from a laceration in the mucosa at the junction of the stomach and esophagus. This is usually caused by severe vomiting because of alcoholism but can be caused by any condition which causes violent vomiting and retching such as food poisoning often presents as an episode of vomiting up blood after violent retching or vomiting, but may also be noticed as old blood in the stool and a history of retching may be absent. In most cases, the bleeding stops spontaneously after 24–48 hours, but endoscopic or surgical treatment is sometimes required and the condition is rarely fatal. PS. You didnt tell us the duration but mostly am assuming its acute or within 24 hrs other possiblities like chronic liver disease complicated with esophageal varices can also present like the main thing is he should do endoscopic evaluation to determine the specfic cause.