The literary technique Frost is using in line 27, and at other points through the poem, is called "hard caesuras," which is an abrupt interruption in the flow of the language of the poem. The technique is being used in this particular poem to indicate points in the story being told when there are unanticipated and sudden, startling breaks in the action.
The word "So" could be interpreted as a halt while the person telling the story has to take a deep breath and control his/her feelings before continuing the story. It could also be read as indicating that, in spite of the boy's pleading, "Don't let him cut my hand off - the doctor when he comes. Don't let him, sister!," observers could already see that the hand was too badly damaged to be saved. In the coldest, most unemotional sense, the "So" could be seen as conveying the attitude that life continues, regardless of accidents or emotions, and there's nothing more to be said or done about it.