Wilson says that some people are little boys forever and they never become men. What descriptions of these men does he give?
Wilson is, of course, referring to the protagonist, Francis Macomber. Wilson is observing how Francis is being bullied and manipulated by his wife, Margot. Francis, a rich American traveling abroad, is weak and fearful. This is why he refers to Francis as a child.
The story really is Francis' coming of age. He has never really lived like a man before. For a brief time, at least, in the story, Macomber is able to conquer his fear and experience life without fear or weakness.
We are exposed to Wilson's thoughts regarding this late in the story as they are about to enter into the brush after one of the buffalo (in this way, Macomber has a chance to make up for proving his cowardly reaction to the lion the day before). Wilson notes, "It's that some of them stay little boys so long . . . The great American boy-men." What Wilson (and Hemingway) means by this is that American males have become softened by money, privilege, and their women. They have forgotten what it means like to face their fears and conquer them.
Wilson notes that Francis "had probably been afraid all of his life . . . But [his fear is] over now." It is this lack of fear that is really the "[M]ain thing a man had. Made him into a man. Women knew it too. No blood fear." And indeed it is Francis' ability to conquer his fear that ultimately leads his wife to (possibly) kill him, for as Wilson notes, "'He would have left you too.'"