For what it's worth, here is an article debunking a supposedly scientific article that claimed the Covid vaccine causes Alzheimer's-like symptoms. So if you run across the original article, you'll at least know it was not a properly done study.
Anyway, the H1N1 vaccine a decade or so ago gave some people Guillain-Barre syndrome, which is not the medical issue the vaccine was trying to solve. The Covid vaccine has provoked an inflammation of the muscle around the heart in some (few) people, and this side effect was caught pretty fast despite the numbers of people who had it being pretty small compared to the numbers of people getting the vaccine. Also, sometimes a physical jolt to the system (like an illness, a vaccination or even being in an accident) has been known to trigger a condition a person already has, that wasn't showing up noticeably. (For example, I once got really a painful knee inflammation after I got the newer version of the shingles shot. As it turns out, I have arthritis in that knee, which had developed over a long time and I didn't know about it. The doctor couldn't say whether the shot triggered the flare-up, but it was clear the arthritis was there well before the shot.)
I realize this doesn't do much to try to find a solution for your father-in-law. If he did get it from the vaccination, that would be extremely uncommon, simply because with millions of people getting the Covid vaccination, if people were getting Alzheimer's from the vaccine, it would be seen. The question is still being asked, though.
In the meantime, try researching "sudden-onset Alzheimer's-like symptoms," rather than searching for a definite link of the Covid vaccine to Alzheimer's. It would be nice if this is something that can be solved, or that you can expect to see ease with time. My dad had Alzheimer's but it developed slowly, and much later we learned we should have been watching his teeth, as bacteria from cavities can be very dangerous in terms of Alzheimer's. So you never know. Good luck to you in your search, I hope it is something he comes out of.
Has he been tested for covid? I ask because covid can cause blood clots all over the place, including the brain. Alzheimer's isn't the only form of dementia. Vascular dementia is caused by blocked blood vessels, something I haven't seen reported from the vaccine but which has been reported from having covid. Otherwise I haven't heard of this problem but everyone is an individual and all medications have side effects that only some get and most don't and sometimes we're really unlucky. As for any place testing for neurological problems, GBS is a problem but I haven't seen it reported much with the mRNA vaccines, it's been more of a problem for the other types of vaccines, but it can happen. Of course, it's much more often caused by covid or the flu or the things you're being vaccinated against. Sometimes with meds the reaction isn't to the active ingredients, it's to the fillers and binders and other things they use that can cause bad reactions. And of course, anyone can turn out to be allergic to any drug including vaccines. Sometimes the body just reacts to the drug that way. It's often hard to find someone to see who is involved enough in the weeds of this stuff to know exactly what's happening. You might try contacting CDC or Fauci's office or the office of a vaccinologist because they would be looking at everything really closely as they aren't spending most of their time tending to patients. Best of luck.
I've heard of various problems related to the people have had a reaction to the vaccine.. Some people have reported dizziness. The Vaccine manufactures are exempt from lawsuits. Even if you can prove that this happened because of the vaccine in the long run all that matters is taking care of your dad. Good luck
Covid can accelerate the timeline for manifestation of Alzheimer's patient's (including previously undiagnosed) problems so perhaps he coincidentally got asymptomatic Covid shortly before or after the vax, although the neuro guy might have looked into that. Here is a Sept article. https://www.cnbc.com/2021/08/31/covid-could-cause-significant-rise-in-dementia-cases-alzheimers-group.html
My father in law has a history of Alzheimer's on his side of the family. He was starting to show very early signs, very mild before the pandemic, nothing crazy or really bad just occasionally misplacing things etc. so mild in fact you wouldn't even have thought it was anything other than normal age associated memory degradation. When he got his first dose of the vaccine within a week it was like someone flipped a switch and he was asking me if I had ever met the neighbors before - I've lived here for 8 years and gone to bbq's at the neighbors with my in-laws many times and interacted with them frequently. After the second dose it got worse and after the booster we are now at the point where he sometimes can't recognize his own house, and is having frequent hallucinations/paranoia and has had his driving privilege's revoked. Before the vaccines he was very active and could function just fine without constant supervision. Personally I don't think it's a coincidence at all. While I don't think the vaccine itself CAUSED the Alzheimer's, I think it certainly exacerbated it and made it progress much more rapidly. It's soo sad to watch someone decline so rapidly, knowing possibly that it was nudged along. I feel like he was robbed of a few more good years and now I'm watching this poor man fade away. I'd love to know if others have experienced similar things with regard to memory loss, rapid onset of Alzheimer's symptoms etc. post injection with their loved ones.
I'm sorry, it must be heartbreaking.
I experienced my dad having a similar rapid decline, going from pretty normal and lucid to almost not there, in a short time. In his case, he had three teeth that were so badly decayed they needed to be pulled -- his apartment even stank of his rotten teeth. We think bacteria from his teeth caused his sudden Alzheimer's downturn. (It was before Covid.)
No matter if you can prove it's the cause or not, did you report this on the Vaccine Adverse Reporting System ("VAERS")?