@jackmiaw Rather than just giving his body to them, they are parts of his body now. We saw how the wolf was able to move independently as his severed hand. He probably just straight up isn't human anymore, and might be more or less immortal so long as he keeps them interested.
That might not sound like such a bad deal and all at first blush, but he did all of this for his family... who would die of old age long before he could in that state. After they're gone like that, he'd pretty much just be a vessel for the three spirits to do with as they please so they can interact with this world through him. "Immortality is actually a curse" wouldn't exactly be a new thing in storytelling, either, so I could totally see this being one way things could go here.
Then again it could be as simple as him having become a vessel for them at all, rather than a long lasting immortal one. He pretty much only has any agency to do as he wishes for as long as it pleases the three spirits to continue allowing him that, he did offer them everything. They can collect that as they please. Either way is probably a bit further than any of the other contractors have gone, giving up stuff like an organ or a sense. He offered his entire self.