Most of the time, the one who isn't Arthur is referred to as "old man," but once as "withered man," and another time as "weak old man." It's a little confusing when he's referred to as "withered man" as it's close to the beginning where Arthur has just been introduced. Also, I don't know that "weak" is necessary, as the decay of the house seems indicative of that.

Also, I found "gunky" to be a bit distracting. Nitpicking aside, I liked the story though I thought it was odd that the old man should be the only flesh and blood person left. I got the impression that no one else had ever chosen not to undergo whatever it is that Arthur has. I know the story doesn't say that, but that's the impression I got.

Perhaps some more detail about Arthur's form. At one point, we are told that he is irradiated. If that's so, isn't his presence detrimental to the old man's health? If he's intangible, then is he some kind of human-shaped field of radiation? Perhaps fewer details about Arthur's form would be an even better way to go, but I think some details are necessary so that it's clear that he isn't a ghost.