Other mistake: When we first see Jeff Bridge's character on the elevated subway, the car is jostling around in obvious motion. However, the scenary visible through the rear door window of the train is clarly not moving in response to the train. The city background is not moving farther away. The scene quickly changes angle and the background now begins to "move" in relation to the progression of the train car. (00:14:45)
Other mistake: When Prot visits the doctor's house, he thanks the wife and the camera is zoomed into his sunglasses. The reflection of his glasses shows the doctor looking at him, but the doctor is standing behind him. (00:53:40)
Answer: He rejects it for two main reasons. First, each of the items you mention have possible, even if unlikely, explanations. Some people have strange or no reaction to certain drugs (for example I have almost no response to any painkillers). People who have had their corneas replaced with artificial lens can see near ultraviolet (though nowhere near 300-400 angstroms). The sheriff described Porter as being very bright, and he was in to astronomy, so while a great stretch, not impossible he somehow formulated the information he presented. The second reason, building upon these, is Occam's razor. As a person in the sciences, Dr. Powell is driven to believe things have a reasonable explanation, even if we don't currently know what it is, and thinking Prot is just a bright and unusual human is a more reasonable belief to him than believing Prot is an alien possessing a human's body.
jimba
Just remarking, there's no comparison of painkillers and psychiatric drugs. Thorazine and Haloperidol (Haldol) are both powerful anti-psychotic drugs with numerous side effects. Prot is immune to Thorazine and Haloperidol (as well as alcohol), which is more than extraordinary, it's otherworldly.
Charles Austin Miller