He is the Second Officer.

The Second Officer sits at the Flight Engineer station in the cockpit of the older airplanes which required an F/E. F/E used to have two trips on their shoulder/sleeve. Second Officers generally wore three stripes because they were pilots who were currently assigned to the S/O position, not professional flight engineers who weren't pilots.

The First Officer is the pilot sitting in the right-front pilot seat. He was called the co-pilot in the old days. The F/O wears three stripes.

The Captain is the pilot sitting in the left-front pilot seat and is the pilot in command. The Captain wears four stripes.