The "him" refers to a dead soldier whom the speaker and the speaker's comrades gather around in the snow. They want to keep the body warm and sunlit to preserve its dignity. Keeping the dead body warm might seem pointless, but it reminds the soldiers of the sun's positive effect on him throughout his life, which the speaker briefly recalls.
From the lines "Gently its touch awoke him once, / at home, whispering of fields half-sown," readers can gather that the fallen soldier may have been a farmer who awoke each morning by the sun's light, which reminded him to finish planting seeds in nearby fields. This recollection of the soldier's past humanizes him, informing readers of a time when he didn't have to be a soldier and risk his life each day.
Notice, too, that the sun has a "touch" as well as "whispers." These two features personify the sun, as well as convey gentle imagery, making it seem almost human in the way it interacted with the soldier when he was alive. Even though the speaker, having seen many other soldiers fall in combat, knows that the soldier will never wake up, the act of moving him into the sun seems the closest thing to an attempt at revival, even if it is ultimately futile. If the sun woke the soldier each day throughout his life, "gently" touching him and "whispering," why might it not do the same now?
The poem also rearranges stress to capture these features. The poem begins with an instruction, and its first line fittingly uses two trochees:
Move him | into | the sun—
These stressed syllables force the reader to pay close attention, as if the reader were the one being instructed to move the dead soldier. At the same time, this line's third foot indicates that the poem will probably follow an iambic meter. And it does: each stanza begins and ends with an iambic trimeter, while the intervening lines are all in iambic tetrameter.
Line 2 then also begins with a trochee ("Gently"), as if the sun is softly pressing down on the meter. Similarly, line 3's second foot is a trochee or dactyl, depending on how it's read:
At home, | whispering | of fields | half-sown.
"[W]hispering" can be read as having two syllables or three. Either way, its initial stress captures the excitement in the image of the sun whispering to a person.
These first three lines also reveal a larger occurrence throughout the poem, which is that of pararhyme. The words "sun" and "half-sown" share an ending consonant, while their stressed vowel sounds are different. This trend occurs throughout the remainder of the poem (and was commonly used by Owen). It conveys a kind of out-of-tune, partially haphazard sound that mimics the dissonance of the battlefield. If every line contained perfect rhyme, the poem would perhaps be too harmonious for its content.