The premise of Saya no Uta is similar to the Buddhist view of world and I believe that Fuminori Sakisaka's situation is described in Buddhism.

It's an extremely complex topic but if very highly simplified Buddhism believes that the whole world is shaped by consciousness and there's no world outside of consciousness. Two humans look at the "same" object but perceive it differently due to their karma(for example one associates bright sunlight with fun, the other - with sunburns and irritation).

This is more radically applied to how beings from different realms look at things. For example let's say that a being from hell, a ghost, a human and a deva(heavenly being) walk up to a river and look at it.

A being from hell would look at the river and see it full of painful acid. A ghost would would see it full of blood and pus. A human would see it to be just a muddy water. A heavenly being would see it a as a crystal blue nectar. They're looking at the same thing but perceive it radically differently due to karma.

Fuminori Sakisaka is trapped in pretty the much the classical depiction of the ghost realm(it's properly called hungry ghost realm). Except that in Buddhism even Saya wouldn't be normal and Fuminori's body and mind would be deformed and he wouldn't been able to interact with humans.

Though to move between realms according to Buddhism one needs to either die and be reborn or be an extremely advanced meditator-saint-master who can manipulate their perception in the deepest layers of meditation and mentally move between these "layers" of perception. Neither of those applies to Fuminori so that's the other difference.

Yeah Idk maybe I'm digging too deep or in the wrong direction but I think there are some parallels to Buddhism in VN.