A Google user
The book and the movie are really about human potential right here on Earth. The writer, Brewer, is using another planet to stand for what his ideals are for Planet Earth. The main character, Prot, talks about crime and punishment, right and wrong, reproduction (which should be done very carefully), how every adult should be responsible for taking care of children, and how everyone should have the ability to travel freely on this planet, this Earth, to get to know other cultures and people.
All of the references to the "other" planet, K-PAX are there to keep the reader's interest. It is science fiction after all.