The real explanation about the ending of K-Pax ****spoiler alert******
I think the real meaning of the movie is NOTHING.... yes, that's right. The director or the writer couldn't deliver a convincing finish because that was not actually possible after giving Undeniable evidence supporting both alternate theories (one is that Spacy was a alien and another is Spacy was a Schizophrenic.
Evidence of Spacy being an alien
1. Non responsiveness to high dose of drug
2. Giving accurate description of the orbital system before anytype of publication about those things
3. Most importantly, detecting UV light, which is not at all possible
There are some minor evidences also for this theory and all taken together, they can not dismissed to be a mere co-incident. So the director had no way to depict him as a schizophrenic in the conclusion
Evidence of Spacy being a Schizophrenic
1. Do not eat meat and porter had a story to hate meat
2. His "going out of control" due to in Jeff's house and the reason behind it supplied in the movie
So, finally, the directer had to leave the question open about the identity of Spacy. That is why, the ending of the movie is very weak. The makers just couldn't cut away all the lose ends.
I still give the movie 8 out of 10 because except the last few minutes, it was a 10 on 10 material.