You haven’t said how he’s likely to feel now - or how he felt after the Hallowe’en sitcom.
There’s huge range between actual trauma (which I know you haven’t said) and one nightmare and a “urgh” shiver feeling about it if mentioned.
People seek out horror and other emotive genres for a reason.
Her first horror movie was at another kid’s sleepover - it was ever thus!
It was months later that she told me it had given her nightmares - because she didn’t want to be told off. Of course I told her she wouldn’t have been told off so badly that it wasn’t worth the comfort. But actually - she didn’t need comfort.
In the moment, she woke up scared. But quickly knew why she’d had the nightmares. Even revelled in it - her and her friends apparently bonded over how hard they were to watch it, and having had nightmares was something to boast about, not hide. Now years on they’ll have a sleepover here and all laugh, “remember when we couldn’t sleep after that film at Lily’s?!”
Not everyone enjoys that genre, but I would say that no-one seeks out a genre that has no emotional impact on them. That would be dull!
So even if he was more sensitive than average to it - consider whether that’s actually a bad thing. It might not be. Even if he doesn’t come to enjoy the impact of a nightmare, he may be unbothered by it. One night of poor sleep and a <shudder> “not watching that film” and move on, may be all the actual impact anyway.
Resilience isn’t not being scared. It’s being scared in the moment then moving on unscathed. You can’t force resilience, but you can give it space to develop by keeping everyone’s reactions proportionate.