Weather is expressed in different ways in different languages. The fact that English, like many other European languages, uses a mysterious “it” as a subject to say what’s going on is actually the outlier. More languages use a formula more like “rain falls, snow falls, sun shines, etc.”

So you tell him the “it” stands for “the weather” although that isn’t true. You could more truthfully say it’s a convention and English sentences need a subject. And then you add that “is raining” also transports the idea that it is in the process of happening right now. Don’t question it, accept it.

Learn a bit of Russian. That language is full of colorful images, irregularities, and inexplicable grammar. More so than English, probably. So you can put him in his place when he complains. Like, dude, y’all don’t even know what blue is!