In this case, our middle-aged protagonist discovers that he's losing weight. Not mass, weight. Steadily and perhaps even increasingly rapidly, he's gone from 240 pounds to 210 pounds without looking as if he's lost any weight. And anything he carries or wears loses ALL its weight. A recognizable medical condition, this is not.
However, unlike the vaguely similar Thinner, Elevation is not a horror story. It's a quieter fable of smaller kindnesses and redemption spurred by that weight loss. I'd compare it to Ray Bradbury if King were a poetic writer like Bradbury. In this case, though, King's own dedication -- to genre great Richard Matheson -- seems apt, at least for Matheson in his quieter moments.
Rod Serling's Twilight Zone would also be an apt comparison, with one Changed Premise illuminating the good parts of the human condition as well as the bad. Think "A Passage for Trumpet" or "In Praise of Pip," two gentle, sad TZ episodes starring Jack Klugman. And a middle-aged Klugman would actually make a good fit for our protagonist!
It's a slight work but an enjoyable one, and it's not going to take you long to read. I'd almost swear that an embattled lesbian couple in Elevation may have appeared in the first draft of King and his son's Sleeping Beauties before being cut. They're embattled because small-town Maine isn't ready to patronize the restaurant of two openly gay women. Or is it? Recommended.