Looks like he had a stomach ulcer that perforated, i.e. broke open. This allowed his stomach contents to pass into his intra abdominal cavity leading to a body wide infection. It appears that they surgically tried to correct the perforation, but the patient died post op from pneumonia. The pneumonia could have been a result of the first infection, or something he picked up in the hospital after surgery. Alcoholism can cause ulcers, but they can also form on their own.
If his stomach ulcer perforated today he would be in serious life threatening condition. His survival odds would be better due to quicker medical intervention, cleaner hospitals, and better drugs.
General Peritonitis - Inflammation of membrane lining abdominal cavity
Perforated Gastric Ulcer - An gastric ulcer is an erosion of the mucous membrane in the stomach. Perforated means that it ruptured, i.e. the stomach split open.
Abcess General Sepsis - Sepsis is a full body infection. Not sure what the abscess refers to in this case.
Pneumonia–Post Operative–/Lung - Pneumonia is an inflammation of the lungs resulting in an inability to breathe. A very common cause of death to people with weakened immune systems.
Although, today, if his stomach ulcer had been determined to be the result of H. pylori bacterial infection (helicobacter pylori), a regime of proton pump inhibitors and antibiotics should have stopped the ulcer, the perforation and other complications.