story identification - Title of a book about a young boy who merges with an alien amoeba

I'm looking for the title of a book from the 50s/60s about an alien amoeba who merges with an adolescent boy, making him immune to sickness and giving him a near-instant healing factor in exchange for the boy's help in locating a fugitive alien that crashed on Earth at the same time.

As the story progresses, they learn that the fugitive alien has merged with the boy's father, who works at NASA, and is using the father's involvement in the rocket program to make his escape.

Any help is appreciated!

Answer

This seems likely to be Needle (1950), by Hal Clement.

From the Wiki description, we see many of the elements described.

An alien symbiote, not actually an amoeba, but sort of close in appearance (just much larger), and the presence of a fugitive.

The Hunter, an alien lifeform (when not inside another being, resembling a four-pound green jellyfish) with the ability to live in symbiosis with and within another creature, is in hot pursuit of another of his kind. Both crash their ships into Earth, in the Pacific Ocean, and both survive the crashes.

Adolescent boy

The Hunter makes its way to shore (its erstwhile host having been killed in the crash) and takes up residence in the nearest human being it can find (as it turns out, fifteen-year-old Robert Kinnaird) without letting the human being know.

Healing factor (after the alien has later revealed himself to the host):

Upon arrival, the two begin to seek out their quarry. Bob is injured by an accident. The Hunter is able to hold the wound together, but he can't stop Bob from limping, and Bob is sent to the island doctor.

More on the healing factor, and the fugitive being Bob's father:

The Hunter is able to solve the riddle by observing the behavior of the island people. Bob's father, known for his attention to detail and safety, has been taking amazing risks. He is at least unconsciously aware that an accident will have no ill consequences. The quarry resides within him. The Hunter confirms this, and Bob and the alien have a new puzzle—how to get the alien out of Mr. Kinnaird's body without harming the man?

I can't find any cites for the father being involved in a NASA equivalent or rocketry in general, and my memory of the book is far too old to provide confirmation personally, but it seems pretty close all in all.