The phrase implies that Ezer had already repaired a section and now volunteers for another.
As he has not been mentioned before, we are best to understand that he was not the leader (or foreman) of the earlier crew (but a participant) and now becomes the leader. The term appears again in relation to other builders in Nehemiah 3:20, Nehemiah 3:27.
19 Next to him Ezer the son of Jeshua, ruler of Mizpah, repaired another section opposite the ascent to the armory at the buttress.