"Metrical Feet – A Lesson for a Boy" (Title)
The poem is subtitled a "lesson for a boy." Note the fact that it isn't called "a lesson for my boy" or "a lesson for my son," but "a lesson for a boy." The generic "a" suggests that this poem might be useful for any number of little boys, not just the specific one Coleridge had in mind. Do you think he wanted this poem to have a wider audience than just his son?
From long to long in solemn sortSlow Spondee stalks, strong foot! […] (2-3)
These lines almost sound like something a teacher would say. Imagine your teacher reciting them at the front of the class.
Slow Spondee stalks, strong foot!, yet ill ableEver to come up with Dactyl's trisyllable. (3-4)
The phrase "strong foot" is neat. First, it refers to a literal foot because the poem portrays the spondee as some kind of stalking creature. But it also refers to the spondee as a unit in a line of poetry – a foot. This little pun offers itself as a fun and engaging method of instruction, and a new way to think about meter.