I think what they might be trying to get at is the concept of an “active placebo.” That is, a treatment that does something, but can’t treat the condition that’s being studied.
I disagree that psilocybin is any sort of placebo, but I think this captures the intent of the grandparent comment.
I hope you are able to take at least some comfort from actually knowing the cause of your son's illness. Some people aren't so lucky.
I had my own medical struggle while in the middle of grad school. Mid October a few years ago, I contracted a rather nasty respiratory infection. Among other things, I was severely fatigued and needed to sleep for extended amounts of time (think 12+ hours a day). The excessive sleeping didn't worry me while I was busy hacking and coughing, but when the respiratory symptoms had gone away a couple weeks later, I was still left with the extreme fatigue and hypersomnia.
Normally, I'm not one to run to a doctor at the slightest sign of any illness, but I began to get worried a week or so after the acute infection passed and I was still sleeping a minimum of 12+ hours a day. (I should emphasize, too, this really was a minimum, not an average -- I'd typically wake up around 8 am, go to class and do other things until around 2 or 3, come home and fall asleep around 4 PM and wake up and do it again the next day. One weekend, I think I was awake for a total of about 8 hours over the two days.)
I went through the usual struggle anyone who has anything that's at least mildly rare goes through, and, after visiting a sleep neurologist, I got my diagnosis: post-infectious idiopathic hypersomnia. I then knew this was a problem that was going to take months or years to resolve, and, for a while, I thought my life was effectively over.
But, the real kick in the teeth was that word, "idiopathic." There's a great line on an episode of House where one of the fellows suggests a diagnosis of idiopathic something-or-other, and House responds "'idiopathic,' from the Latin meaning we're idiots because we can't figure it out." So, there I was, with a diagnosis that amounted to "you sleep a lot and we don't know why," and the only treatment available was basically stimulants to treat the symptoms (which didn't work well at all -- I slept less, but I was walking around like a zombie by early afternoon every day).
Looking back, I consider myself lucky that it happened while I was in school, since I could have easily found myself unemployed and broke if I were in less flexible circumstances at the time. I'm also quite lucky that it resolved itself within about 9 months. I effectively lost a semester of grad school that I had to make up later, but I recovered, and it should never recur.
I don't know if what I had is what would be considered a "rare" disease by the strict definition, but it's rare enough that your average primary care physician might not ever see a case in his or her life, and a sleep specialist might see a handful. It's rare enough that there don't seem to be any studies or research available on the condition, on top of the obvious difficulties of studying people who effectively have to sleep 1/2-3/4 of the day.
So, back to my original point: my best wishes to you and your son, of course, but please take comfort in the fact you know exactly what is wrong.
I disagree that psilocybin is any sort of placebo, but I think this captures the intent of the grandparent comment.