Move him into the sun—
Gently its touch awoke him once,
At home, whispering of fields unsown.
Always it woke him, even in France,
Until this morning and this snow.
If anything might rouse him now
The kind old sun will know.
Think how it wakes the seeds—
A literal interpretation is that naïve comrades believe the sunlight will awaken their friend, because it has done so before. However, young men who grew up on farms or in rural settings would have been familiar with the slaughter of livestock, and with the high infant and child mortality rates. So death was familiar, a part of everyday life. Therefore, the attempt to revive the fallen soldier can be interpreted as a symbolic act; an attempt to revive hope in the face of the horror of war and death. 
 By using snow as a  metaphor  for the war and the soldier for its fallen victims, Owen could also be questioning whether or not human civilisation will be able to awake from the savagery of WWI. Note also that “morning” can be read as “mourning”.