Hello to you; I'm glad I saw your post.
Several years ago when my son was 13, he was in a terrible accident and was in a severe coma and was put on a vent. They had to use an adult vent and got it in too far at first and then had to pull it back out of the main stem bronchus, if I recall the terminology correctly. One of the first things that I noticed when the vent was removed a few days later was that his voice was very hoarse.
A few weeks later, he was eventually referred to a specialist, an otolaryngologist which I'm probably misspelling but that's an ear and nose and throat specialist, and that doctor dropped a scope down his nose (which was miserable for my son) and said, "Yup, there's a small growth on his vocal chord." I asked the doctor if it was big enough to make my son's voice so terribly raspy and hoarse and he said that he was surprised that such a small growth could make such a big difference but it could happen.
Unfortunately, that dumb doctor didn't drop the scope down low enough to see the much larger growth on Levi's windpipe.
I'll spare you the details but this thing exacerbated and then manifested itself by almost completely cutting Levi's airway off three months later. It happened early in the morning and we rushed him to the nearest (ridiculous excuse for a) hospital.
They misdiagnosed him the entire day, first in the ER and then after emergent admittance, as being asthmatic until one smart doctor recognized the huge growth, which had flipped up like a flap cutting off my son's airway, where it had been hanging down previously. I don't know how they misdiagnosed him so badly. He could exhale fine but could barely inhale and the breathing tests they performed clearly showed this. Also, an x-ray they did in that pitiful excuse of an ER clearly showed the spot where the growth was. I'm digressing.
Anyway, he was sent by ambulance the next day to another (much better) hospital where a much better ear and nose and throat surgeon successfully removed it in emergent surgery. When my son awoke, his hoarseness was completely gone. That surgeon told us that the growth occurred on a nerve in the windpipe that is not uncommonly damaged by vents, something he was making a personal study of; and also he had not frequently seen such a growth occur from that nerve area like our son's experience.
I certainly hope this is not the situation for your son, but thought I should share our experience.
There is a retired doctor of that specialty who posts here, usually in the anti-coagulation forums, although I don't recall the name he goes by. Perhaps you could search him out and send a PM to him for information?
Best wishes to you. Please let us know what you find out? Take care.