The problem is that viral videos capture attention way better than walls of text, so our brains prefer that easy route! But this means your son is getting scattered pieces of history without the real context - maybe the video about World War II is exciting and accurate, or it's a crazy oversimplification or even wrong, and he doesn't have the tools to tell the difference.
Young people aren't consciously choosing videos over books, it's more that social media works like an attention drug - they're quick dopamine hits, easy to consume while you're eating or waiting for the bus. Books and Wikipedia require effort and concentration. The real challenge is educating your son to use videos as a starting point (yeah, watch the interesting video!) but then dig deeper with solid sources, like full documentaries or reliable history articles.