The poem's opening line introduces Miniver Cheevy as someone who has been disdainful of life ever since he was a child. The phrase "child of scorn" suggests that Miniver is and always has been highly critical of the world, though it's not yet clear why, exactly, he has such a disapproving outlook on life.
Building upon the conception of Miniver Cheevy as an unhappy, grouchy man, the second line suggests that he does not live a prosperous life. He is "lean," suggesting that he's underfed and poor. In turn, the speaker subtly associates Miniver Cheevy's negative outlook on life with hardship and struggle.
At the same time, Miniver seems too distracted by his scornful attitude to pay attention to his health or well-being, an idea implied by the fact that he spends his time "assail[ing] the seasons" while he gets poorer. To "assail" means to attack or assault, and as such this phrase hints that Miniver hates the passage of time—an idea that will become clearer in the second stanza. For now, though, it's evident that Miniver is an unhappy man with a pessimistic worldview that keeps him from properly taking care of himself.
The opening stanza also establishes the poem's interesting meter. The first and fourth lines follow iambic tetrameter, meaning they contain four iambs—metrical feet made up of an unstressed syllable followed by a stressed syllable (da-DUM). While these lines thus have eight syllables in total, the second line in each stanza actually has nine—ending each time with a final unstressed beat. To get fancy about it, this is something called catalectic iambic pentameter; it leaves the reader hanging, expect that final DUM.
The fourth line in each stanza is then also catalectic, featuring an incomplete line of iambic trimeter. This just means that it is made up of two iambs followed by just the first half of a third iamb. This might all seem confusing, so take a look at lines 2-4 to see how this changing meter affects the pace and feel of the poem:
Grew lean | while he | assailed | the sea- | sons;
He wept | that he | was ev- | er born,
And he | had rea- | sons.
Line 3 is a perfect line of iambic tetrameter, containing four iambs. Lines 2 and 4, on the other hand, end with those dangling unstressed beats, those strange half iambs. This makes the stanza feel a bit off-kilter and uncomfortabe. Although the speaker claims that Miniver has "reasons" for being so scornful, the incomplete rhythmic quality of this line makes the speaker sound somewhat unsure and skeptical. As a result, it feels unlikely that Miniver actually has good reasons for being so negative.
This stanza also establishes the poem's ABAB rhyme scheme: the first and third lines rhyme with each other, as do the second and fourth lines. The second and fourth lines also feature what's known as feminine rhymes, or rhymes in which multiple syllables rhyme with each other. For instance, the first syllable of the word "seasons" rhymes with the first syllable of the word "reasons," while the final, unstressed syllables of each word also rhyme with each other: "seasons" and "reasons."