The word "narrow" contains short letters and the word "fellow" contains three tall letters and this connotes the short side-to-side movement of the snake while the long letters describe his straighter movement. The alliterative use of the 's' also resembles the hissing sound the snake makes. The letter 'S' itself resembles a snake. The repetition of "L" and long "O" sounds literally sounds "mellow." The image of the snake is calming but also surprising ("His notice sudden is"); so, the image of the snake makes the speaker relaxed but also a bit uneasy.
Viewing the poem as figuratively sexual, the snake is clearly a phallic symbol. And as the snake comes through the grass, it "divides," with the letter 'V' in "divides" appearing as a vaginal symbol. So, it is not just the sounds of the words and letters; it is also the appearance of the letters that adds to potential meaning.
The final two lines have more disjointed sounds. Disjointed sounds are less graceful, less calming than the alliterative phrases. This illustrates the speaker's fear of the snake. (Consider this in the literal or figurative interpretations of the poem.) The phrase "Zero at the bone" does use assonance (repetition of the long "O" sound), but it comes across as coldness. It is an icy feeling of fear which means being chilled (zero degrees) to the bone. The "O" sound is in the word "zero" and the letter "O" is nearly identical to the number "0."