The soldier boy received an injury in his chest the previous night when the explosion took place and his fellow soldiers died. It cannot be ascertained whether he knew that he would die or not. But chances are that he was not actually aware of his death.

There are a number of points that can go in support of that statement.

The soldier boy called the narrator and wanted some water. So it is probable that he expected to get better and finally walk away.

A sip of water on this morn would surely do me good.

The soldier boy also repeatedly refers his would to be “a small pain” in his chest. Though it has some deeper implications, it may imply that the boy was confident enough to get well soon.

Another expression, “I count myself more lucky than the rest” may suggest that he was not actually aware that he was going to face the same fate as others.

And this line: “I think, before I travel on, I’ll get a little rest…” surely expresses his confidence, or rather hopes.

And, after all, no man can ever know for sure that s/he is going to die. And even if one knows, everyone hopes till the last. So, I would argue that the soldier boy was more unaware than aware of the fate that he met in the end.

No, the soldier boy did not know that he would die of that injury, that small pain in his chest.