Irony exists on several levels. On the linguistic level, the fact that "the man" of the title calls Jensen "boy," reversing a standard linguistic practice of whites towards Blacks, is ironic.
On the level of physical action, the man puts his hands around Jensen's neck, in a gesture that Jensen finds threatening, but he does so just to measure Jensen for shirt size for a gift. To threaten while giving is ironic.
On the level of plot, the fact that Jensen warns Lena about the man, only to have them get married, is ironic.
what is the point of view for this story?