Here's the issue. The player character in an upcoming game is in a sorta-afterlife.That's not explicitly laid out, but it's hinted at in how other characters talk about the world and their pasts. This primitive world is a place where the gods, or rather the celestial bureaucracy that may have replaced them or may have fabricated them in the first place, dump all the people they don't know what to do with. Mostly, it's people who did something truly horrible, but were in such extraordinary pain that lashing out in a horrific manner was inevitable and their judge just couldn't punish them for it and had to make them disappear.

The PC is a small child. They are probably 4-5 years old, maybe a year younger or older. The only hinting to their backstory comes from one character who appears a total of twice in the game, once as the first NPC they meet and once as the final boss. I'm not getting into what that backstory is because this is about technique. He's a middle-aged man, who saves them from the lethal sunlight, and he comments before he dies that they said some things that disturbed him. Later when he appears as the final boss, notably with his face covered, he lays it out in more detail, and several other times throughout the fight. That's all I have, him laying it out at those times.

Here's the issue: This is rather clumsy. I would much prefer a method of delivering this information that isn't as hamfisted as an NPC revealing all of it all at the end. The infodump at the end can be kept for the slow folks, but what can we do to hint at it earlier so it doesn't seem it it was all pulled out of the guy's ass?

Are there (or could there be) items or reflections of the character's life in the afterlife? If your afterlife were to some extent effected by the life of the character you could use the world design, items, etc. to provide additional hints.

Is the final boss of significance to other characters and the world in general, or only to the PC? If the former, perhaps his impact on other characters or the world itself could provide some clues somehow?

Other techniques commonly used for this sort of thing include flash backs, dream sequences, found diaries/photos/possessions/etc. I'm unsure if any of those will be of use, but that's the basic options I can think of, I'm sure others more experienced at writing may have more ideas or might be able to be more specific.

Are there (or could there be) items or reflections of the character's life in the afterlife? If your afterlife were to some extent effected by the life of the character you could use the world design, items, etc. to provide additional hints.

No, it's only a sorta-afterlife. It's complicated, but it's a physical place.

Is the final boss of significance to other characters and the world in general, or only to the PC? If the former, perhaps his impact on other characters or the world itself could provide some clues somehow?

He's significant to the player only because he saved their life at the beginning of the game, jumping into the sunlight to carry them from under a tree into a cave. As he died in the process, this is one of the larger hints as to the nature of the location. That is, respawning isn't just a gameplay mechanic, it's actually part of the world.

His significance to the world is quite a bit stronger. He's the local lord, from the stronghold that takes up the entire west side of the map, but the stronghold is falling apart and the church has committed atrocities against the surrounding settlements. The captain of the lord's military company opposes this and has tried to expose these actions to the lord, but he trusts the pontiff too much and now the church and the military are fighting a clandestine war. The lord is aware that the church and company are fighting something but not that they're fighting eachother, and the captain and pontiff are both lying to him about it. The entire region is in its current sorry state because of the ambition of a religious zealot and the lord's inability to accept that his long-time friend has become such a fanatic.

I'm certain that's way more information than you need, but there you go.

Other techniques commonly used for this sort of thing include flash backs, dream sequences, found diaries/photos/possessions/etc. I'm unsure if any of those will be of use, but that's the basic options I can think of, I'm sure others more experienced at writing may have more ideas or might be able to be more specific.

Yes, but there's a few issues here. While flash backs and dream sequences are possible with the structure of this game, they can't be done in such a way as to guarantee the player will find them. Diaries, photos and other items could only be included in such a manner as well as none of them would exist here otherwise. I have thought to use item descriptions to hint at it, but they'd be rather subtle and not everybody reads those.

Yes, but there's a few issues here. While flash backs and dream sequences
are possible with the structure of this game, they can't be done in such
a way as to guarantee the player will find them.

That might be okay. If you think it isn't, you can design it to be okay.

Unfortunately, it's part of the nature of a hint that not everyone will pick it up. We can try to mitigate that by repeating hints or making them more obvious, but no matter how obvious some players might still miss it, and at a certain point becoming too blunt and "in your [the player's] face" becomes undesirable - we are after all trying to find methods to avoid reliance on bluntly telling the player outright.

What sort of actions will the player take in the game, and do they have a voice at all or any way of expressing their self not directly controlled by the player? If it's an option at all another approach might be to have the character make comments like "this feels familiar" if they're carrying out actions similar to that back story.

This specific problem seems very related to the recent discussion on "telling a story vs. exposition", so if you haven't already seen it that discussion might also yield some ideas, although being more theoretical it doesn't go into a lot of specifics.