Was the pillowcase ever found? and they assumed amnesia because there had been other cases of it, why? Why were people suddenly contracting amnesia? What did the fish in the bed mean? And what was he doing in the months before his disappearance? also, there is a tunnel that runs from that dorm to another, was it ever checked?
From what I can gather, amnesia is a fairly rare condition and always has been. But it is often put forth as a theory when a person goes missing if there is no obvious evidence found of foul play. I think it is emotionally easier for someone to think that their loved one has suffered from amnesia than it is to think that their loved one deliberately walked out of their life or that their loved one has died.
For at least some people, it is so hard to accept that their loved one might be dead that they come up with very unrealistic theories about the disappearance.
For example, there is a river that runs through my home town that has a dam across it and a double bridge that is located just barely on the upstream side of the dam. It's not a huge dam but the force of the water going over it is incredible. There is certainly no way that a human being can survive being washed over the dam, the force of the water holds the body down to the riverbed until the body gains enough buoyancy from decomposition to start to rise, at which time the current pushes the body downstream (someone goes over the dam once or twice every decade, so it is well known what happens).
In the case I am thinking of, a young man who had been drinking (probably a lot) was walking on the bridge that carries westbound traffic when he saw some friends on the other bridge that carries eastbound traffic. They stopped to greet one another and he decided he wanted to join their party, Rather than walk to the end of the bridge to join them, he decided to climb up onto the rail and just jump to the other bridge. Unfortunately, there is an optical effect with the two bridges that makes it appear the railings on each bridge are only two or three feet apart when the reality is that the railings are close to 12 feet apart. His friends tried to stop him but he climbed up on the rail and attempted the jump. He fell between the two railings into the river and was immediately washed over the dam. His body wasn't recovered for close to three weeks, about six miles downstream of the dam.
During the three weeks before his body was found, his family could not accept that their son had drowned, despite the eyewitness testimony of his friends. They thought he hadn't really fallen in the river or that somehow he had caught himself on one of the bridge pilings and somehow made his way to shore without any human assistance and was wandering in an amnesiac state due to head trauma. And then they accused the friends of making up the whole story or being mistaken about who they saw that night. Their theory was that he hadn't been there at all but had sustained some sort of head injury and was wandering around somewhere outside of town (to explain why he had not been sighted in town at all).
To an outsider, the family's theories were just not realistic or even possible in any way. But for them, it was much less traumatic to think their son was wandering around with a head injury than to think he had drowned and all they could do was wait for his body to eventually be washed downstream where it could be found.