So he cried for her breast and milk. It's hard For fathers to compete with mothers' love. In the dark, mothers illuminate like the stars!
Notice how line 7 begins with "So"? "So" is often used to mean something like "therefore," so at first, it seems like the son is crying for his mother because she is more important than the stars. But really, the son is just hungry and the mother has come in to breastfeed him.
Finally, in line 8, the speaker stops describing the action and addresses the reader. It's like he's turning toward us and saying, "Hey, it's hard being a dad!" It's somewhat of a confession, of sorts, or admittance that he struggles with his role as a father.
He's also admitting that he feels competitive with his wife. Sort of weird, Dad! Your son is starving and all you can think about is how you're competing with your wife for your son's attention? Chill out, why don't you?
In line 9, the speaker alters his comparison a little bit. In the previous stanza, he said mothers were more important than the stars, but here, they are like the stars.
Of course, the poem has to keep up the rhyme scheme and stay true to form, which means repeating lines and images. But Alexie isn't afraid to jazz up those lines a bit, to add some variation.
He's using a cool simile to blend together what's important to the speaker in the poem: stars, son, mother.