He sat in a wheeled chair, waiting for dark,
And shivered in his ghastly suits of grey,
Legless, sewn short at elbow. Through the park
Voices of boys rang saddening like a hymn,
Voices of play and pleasure after day,
Till gathering sleep had mothered them from him (lines 1-6).
In the dark, no one can see you; he can be the hero he was before he lost his limbs. In the dark, he doesn't have to face reality.
"Shivered," in line two, indicates that he is outside and this conjecture is backed up by line three, where the narrator mentions that he was going "through the park".
It is a common practice to sew shut pant legs and sleeves when someone is missing that appendage; this man appears to have lost his legs and a forearm.
The voices of boys and it made him feel sad; they make him remember his childhood. It was not long ago that he was like those boys running around with any cares, but it all seems like a distant memory. The war robs you of your innocence and naivety. He is also probably a little jealous of them, they can still believe in fairy tales and happy endings, whereas he knows that not every life has a happy ending. Soldiers lose their youth to the war.