And yet don't look too good, nor talk too wise; (8)
The speaker introduces a theme that will be important later when the poem becomes more obviously political: balance. To be a good leader, the listener must not be "too" much of anything (too wise, too upright, etc.). Excess is no good.
If you can talk with crowds and keep your virtue, Or walk with Kings---nor lose the common touch, (25-26)
The idea of talking with crowds makes us think of political speeches, while walking with kings suggests passing with social elites. A good leader must be able to get along with people from opposite ends of the social spectrum ("Kings" and "crowds").
If all men count with you, but none too much: (28)
Well this is an interesting political idea. One must be liked ("count with you"), but not too much. As with the rest of this poem, excess of all kinds is bad—in politics and in life.
Yours is the Earth and everything that's in it, And---which is more---you'll be a Man, my son! (31-32)
Okay, so the speaker probably means "Earth" in a general sense here, but in a poem inspired by an invasion? That screams for a political reading. The poem's suggestions are partly an instruction manual for controlling land (cities, regions, countries, the world, the universe, and quite possibly beyond).