I'm looking for a short story
"Danger—Human!", a short story by Gordon R. Dickson, also the subject of this question; first published in Astounding Science Fiction, December 1957, available at the Internet Archive. The story has been reprinted a number of times; perhaps your father will recognize one of these covers.
. . . about a human given immortality or extended life by aliens to figure out how humans work.
"We do this," said the doctor, "not only because we may discover you to be more dangerous than you seem, but to impress you with your helplessness so that you may be more ready to help us. Here you are, and here you will stay."
"And you think," demanded Eldridge hoarsely, "that this's all going to make me want to help you?"
"Yes," said the doctor, "because there's one thing more that enters into the situation. You were literally taken apart physically, after your capture; and as literally put back together again. We are advanced in the organic field, and certain things are true of all life forms. I supervised the work on you, myself. You will find that you are, for all practical purposes, immortal and irretrievably sane. This will be your home forever, and you will find that neither death not insanity will provide you a way of escape."
The aliens, or whoever had him, wanted to figure out why humanity would explode out into the universe and then go back home.
"Briefly, there is a race that has three times broken out to overrun this mapped area of our galaxy and dominate other civilized cultures—until some inherent lack or weakness in the individual caused the component parts of this advance to die out. The periods of these outbreaks has always been disastrous for the dominated cultures and uniformly without benefit to the race I am talking about. In the case of each outbreak, though the home planet was destroyed and all known remnants of the advancing race hunted out, unknown seed communities remained to furnish the material for a new advance some thousands of years later. That race," said the academician, and coughed—or at least made some kind of noise in his throat, "is your own."
He escapes by slowly corroding his prison with his saliva.
Not saliva, stomach acid:
"The hinges of the hatch," he said, "were rotten—eaten away by acid."
"Acid?" the commander stared at him. "Where would he get acid?"
"From his own digestive processes—regurgitated and spat directly into the hinges. He secreted hydrochloric acid among other things. Not too powerful—but over a period of time. . . ."