I would probably not intervene immediately. I know myself and in this type of situations, I usually freeze instead of taking action.
However, I also know myself well enough that once I was halfway through the hallway, I would come to my senses, go back and take that boy with me.
I wouldn't beat up the guy (I wouldn't be able to, nor want to). I would take the kid to a hospital and call the police.
If, for whatever reason, even with an eye witness the guy wouldn't have been arrested within a few days, I would have gone to the police to ask for an explanation. If none was coming, I would have gone to a major, non-local paper with my story.
Shell shock isn't an appropriate excuse for his refusal to act after the fact.