SOLVED. Children's Science Fiction Book I read maybe ten years ago about a Boy And His Dad From Another Planet. [s]

[Zirconia]

I boy is separated from his father. He holds onto his fleece jacket. He makes friends with another boy in the hospital. At one point noise which sounds like an electric piano comes out of his mouth which frightens the other boy. He whispers that he's from another planet.

The father searches for his son. I think he was accidentally shrunk, and so he rides various people and things to get back. At his regular size he walks past the security at the hospital. I remember that he wasn't invisible, he just had the power of being unnoticeable.

He takes his son out and they go to a spaceship. They may have shrunk to fit into it. As they are leaving earth he says that he had gotten used to his body. The son asks what their actual bodies look like and he answers "not grotesque".

[Sue]

One of the 'Ormingat' trilogy by Sylvia Waugh?

[Zirconia]

Yes, it was Space Race. Thank you!

[Kris]

Glad you found your book, Zirconia. Here's the link for Sue's find - Space Race by Sylvia Waugh.