It is actually the Colonel that calls Collins "lad." One of the aspects of this story that it is hard to ignore is the predominance of rank that is alluded to. There are a number of different ranks mentioned in this excellent tale, ranging from the Colonel (the highest rank in the story) to Private Collins, who is the lowest ranking soldier in the story. It is important to remember the importance of rank in stories that explain war, and how lower ranking officers were expected to obey higher ranking officers. Thus it is that the Colonel is able to call Collins a "lad." Let us examine the section of the story were this occurs:
"Look here, my lad," he said, in a pious sort of a voice. "Look here, my lad." Collins was not a lad. "Don't you think that's taking pretty big risks for a little drink of water?"
Thus the title "lad" could reflect the condescension of the Colonel towards a private. However, equally it could also point towards the fact that the colonel feels protective towards Collins, so he uses personal rather than military terms.