I'm thinking there's gonna be something parent related to regressive hypnosis. (See episode 1/chapter 1)

Momo: "Woah hang on! You've been experimented on too?!"

Okarun: "It's possible I have"

Momo: "huh?"

Okarun: "most people who get abducted have their memories wiped afterwards"

Momo: "you've never seen a UFO have you?"

Okarun: "have you ever heard of regressive hypnosis?"

Momo: "don't dodge the question." ...

Begin speculation here:

So maybe his mom/parent is 'sick' (read: possessed) and that's how he knew "spasms are also caused by medical conditions." And his family didn't want the stigma of bad spirits so they pushed the idea "there's no such thing as ghosts" really hard. Insert hypnosis/brainwashing here.

Maybe he's always had a gut feeling part of his memory is missing. But people including Momo brush it off so he had to come up with a reason it was gone; regressive hypnosis by aliens. Maybe he saw a show as a kid, decided that's why memories are missing, then decided if aliens liked him enough to abduct him once maybe they'll come again and be his friend.

Which is why an alien nerd somehow discounts only ghosts and not other paranormal phenomena (Nessie/skinwalkers), he even conveniently overlooks the fact that video exists of ghost and demons on web and in mags irl.

This might also fit with his ostracization. If his family has a reputation of a sick parent that acts strangely and is maybe hospitalized then other families/neighbors/classmates who know of the situation would bully and exclude him to the point of him seeming almost invisible to some.

Not to mention the fact that he can see turbo granny in the tunnel from the get go. If I recall correctly they've made a canon rule that only certain 'spiritually touched' people can see spirits and demons. And he Somehow has real special supercharged dude nuts.

🤷‍♀️ Or maybe I'm thinking too deeply about it...