I watched K-Pax for the first time on July 27th - freaked me out!
*SPOILERS BELOW**
What a coincidence eh!!
I loved this movie, though part of me would be hugely disappointed if Prot was 'really' human because at that point it would totally cease to be a sci-fi movie and more of a psychological drama movie.
I firmly believe though, that he was an alien, given the 'smoking guns' of the uv light and the constellation scenes. Whatever the truth, if there is any, it was quite an imaginative movie with lots of mystery and wonder.