>>17497630
You want the scientifically accepted definition of Deja Vu? There's three reasons that it can happen, and the last one is what gets most people spooked and makes them think they're precognicient.

The first is the simplest, it happens when you live a very boring life where you do the same shit day in and day out. Once you do the same thing so many times your brain catalogues the seemingly duplicate memories as irreverent until such a time as when they might seem relevant. After that all it takes is for you to have a moment of clarity in the middle of doing something you've done before in the exact same way you've done it before and you get a very light deja vu.

Second is more interesting, but it doesn't apply to most people. Some people are legitimatley better at predicting certain occurrences based on only incidentally accused infomation. Google chunking and see if you can find a refrence to it somewhere. For normal people it happens once in a great while, but for some people, such as your's truly, it's rather frequent. It's another form a deja vu that's better described as a very astute and unconscious educated guess, though as such people who experience it frequently are more likley to recognize it for what it is rather than what it's called.

The third and purest form of deja vu is a failure to write memory. What happens is that you see something and instead of logging the immediate memory of that thing in your short term memory like it's supposed to, it gets catalogued in your long term memory instead. When that happens haptic triggers fire off in your head and start screaming at you that you've done this before and the only way that your half-lizard-demigod-supercomputer brain can understand it on a conscious level is to say that you're a freaking psychic.

>>17497870
The tl;dr here and the implied subcontext is that someone of your brother's age is going to be especially prone to the phenomena. Both because he's in a very well regulated environment when you consider school and because his brain is still developing and the late teens and early twenties are where you really do get the most memory writing errors that cause deja vu.

As it happens, the chances are that he'll grow out of it soon and forget about it eventually.

Dreams of the world ending are likley due to physiological stress. Encourage him to keep a dream journal and encourage him to analyze it to try to understand the significance of those dreams. There are plenty of guides online that help with this sort of thing, especially with the anyalisis of the more common and primal dream patterns that humans typically experience.

Polite sage