Answer:
He wants to emphasize the gravity of the boy's situation.
Explanation:
In this poem, Robert Frost mostly uses iambic pentameter. This is the most common pattern in the English language. However, in this section, he deviates from this rule. The reason he does so is because Frost wants to emphasize what a big and tragic moment this is for the boy. The break in the rhythm of the poetry symbolizes the break in the boy's life. His life will be divided in a "before" and an "after" due to this traumatic moment, in the same way in which the poem is divided in two sections due to this inconsistency.
