The metaphor that I will choose to talk about in this poem is the one where the speaker says that triumph and disaster are impostors.
This is really quite effective given the theme of this poem. By saying that these two things are impostors, the speaker is saying that triumph and disaster are things that are not real. He is saying that people must realize that these things are not permanent. Therefore, they must be able to just take them in stride and not let these things "get to" them.
This fits in with the theme of the poem -- the theme that this poem is about dealing with anything that comes in the correct way.
Near the end of his immensely popular didactic poem, "If," Rudyard Kipling writes,
If you can fill the unforgiving minute
With sixty seconds' worth of distance won
This metaphor of the minute as representative of the time of a person's life reinforces the previous lines and the theme of Kipling's: A true man is responsible for his life as, existentially, he creates this life by being accountable for his actions and time.
That the minute is "unforgiving" implies that the metaphoric runner must make use of every second, every moment of his life, in order to have this life("distance" as a metaphor for life) be worthwhile. This wise and frugal use of time is just one of the conditions that make "you, a Man, my son."