I used to have horrible nightmares/night terrors as a child. Demons and all sorts of ****.
When I started to play Doom 1 way back then, crushing demons with gusto and having a great time, my brain learned that I can smash - so in my dreams I started playing Doom with the demons, destroying the hell knights with my doublebarrel shotty.
From there I had the occasional lucid dream where I could "will" weapons into the dream, or spawn them in my hands by envisaging them. Been having fun in dreams ever since.
Often I find lucid dreams causes the simulation to halt, so I would have to take the bullets and manually "will" them into enemies. And if they still keep coming, I have to envisage their heads full of holes since the bullets won't go there. Only then do they die. The moment your brain stops running the script, you have to do most of the physics manually. NPC's sometimes just stops and if you look at them closely, you'll see all sorts of discrepancies - like a kid with a beard, fingers on hands not natural, etc. (Like AI drawings of human hands)
The other plus is that I've grown nerves of steel (at least according to friends and family), so I enjoy all the terror/horror games.
Long story short, let the kid play Doom. (2016 or Eternal should be good, but perhaps drop the difficulty for him as the point is for him to be crushing). Might just work like it did for me.
Still don't believe in ghosts.