The theme of this poem is that the speaker wants his son to grow up to be what he thinks of as a man. To the speaker, being a man involves the sorts of virtues that upper class English people of Kipling's day believed in. These were the values that allowed England to conquer their empire, or so they thought.
To Kipling, these values include being able to maintain confidence in your own abilities because you know you are better than those around you. He also values being very bold -- being willing to risk everything. Finally, I would say he values being able to deal well with all sorts of people -- those above you and those below you.
If you can do all these things, you will be man.