SUCKER PUNCH: My theory - Blondie’s Brother.
If in these dream/fantasy states we believe that every person in our dreams is someone we have seen before, and if we also believe that in these fantasy sequences, because all of the girls are (usually) there, that it’s a collective dream state then my theory can make sense.
In short, I believe the young boy to be Blondie’s brother who has visited her in the mental hospital with his mother recently. Because Blondie is the one who notices him and removes his helmet it’s like she recognises him but can’t place it, because in reality she’s insane, however Sweet Pea is sane, as she only followed her sister, Rocket, into the asylum to protect her, therefore it is reasonable to say that Sweet Pea has seen Blondie’s brother before and vice versa. When he looks at her in the trenches it’s almost accusatory like he knows she is faking her insanity and she looks away guiltily.
Then later, my theory explains why the same boy looks at her so suspiciously as she is about to get on the bus with him, he knows who she is and she is scared she will be found out.
Anyway, that’s my theory which people could shoot to hell by informing me that the boy is played by Eli Snyder, Zack Snyder’s son, who also had small roles in 300 and Watchmen and these two scenes could’ve been just so his son got some face time in the movie in the only two scenes where a young boy could feasibly be, however, I like to believe that even if Snyder just wanted his son in the movie he wouldn’t just do it randomly and that my theory holds water.