This is a long shot, but hauntingly similar to your description.
"Recording Angel" by Ian McDonald was written in 1996.
It is set in Kenya rather than the Amazon.
Earth is being transformed by space-borne nanotech that fell first in Kenya. So instead of terraforming, the story introduces the term "xenoforming".
The xenoforming "stuff" is called the Chaga. It is unstoppable by any human means, and, like the famous Body Snatcher pods, but at the nanometer level, is inexorably covering and changing the entire planet.
The story is written in the third person, but gives the strong sense of being narrated, because it primarily speaks from the thoughts and personal experiences of the woman named Gaby who is a journalist assigned to be the "recording angel" of this unstoppable process.
Mismatch: It does not match the requisite of describing a man's spine being xenoformed.
However, her main correspondent and subject, the other protagonist of the story, is determined to explore this phenomenon no matter what it does to him.
He is a hunter by the name of Prenderleith. She thinks, "He look[s] like the last of the Great White Hunters."
As they observe the advancing landscape turned into alien colored hexagons, he shows her an elephant being xenoformed, with veiny growths and limbs ending in something disturbingly like human hands.
At the end of the story, he runs into the advancing growth. His fate is no different from all other humans on earth; he is simply meeting it sooner.
I read this story in the anthology "Nanotech" edited by Jack Dann and Gardner Dozois in 2000 (which also contains "Blood Music").
I did not know that Ian McDonald had written an earlier "Chaga Saga" for Asimov's Magazine called “Towards Kilimanjaro'.
He also wrote a sequel to it called "Kirinya".
They are also set in Kenya, not the Amazon. However, now I wish to read them. I do not know if perhaps one of them describes a person being xenoformed. That is worth an exploration.