There isn't really a single conventional word that is used, I believe.
Protagonist might work, but it's pretty dry. I'd say in this case, for this story, it might be better to use a more descriptive word, or combination of words that identify the man more directly.
Start by looking at the character's relationships with the other characters in the story - especially the antagonist, his wife. If she's his wife, then he must be her husband. So he is the husband.
But the main character is also a willfully pathetic victim. When I read the story I just want to shake him and say, "Wake up!" (Which I think is maybe the point of the story.)
Calling him one thing over and over, will start to get awfully repetitive, and if writers have one convention, it's that they don't want to be boring. You might pick a couple of characteristics about the man out of the story and rotate through them, using all of them to identify him.