I thought this story was fantastic. Loved it. And, looking through the comments, it seems like a bunch of people missed what I think is the central plot point: The boy did extensive research and discovered that in the alien's world, sibling bonds are unbelievably strong and important. The author went into some detail about that concept and then showed that the alien suffered perhaps the greatest pain possible to his species and lost all of his own siblings. Thus, he has already felt (and may still be feeling) a pain that is so awful that he would do something bizarre and risky just to help someone else (the little boy) from experiencing the same pain.
To me, the entire point of the overpopulation/abortion scenario was to add needed tension into the story. If the little boy wanted the alien to kill some jerk who was actively hurting his sister there'd be no drama in that. But having the boy develop such intense feelings for his unborn sister -- in a way that other people would think strange and would like question the validity of such feelings -- was brilliant because that is exactly how the aliens feel about their own siblings, even if few humans give a crap enough about the aliens to learn that fact about them.
When the story ended I thought, "hot damn I hope this is the first chapter of a book that keeps going!"