Back in the early 2000's, well before most social media sites existed, I had a coworker who shared with me a number of questionable theories, one of which was basically this. He also believed very passionately that plate tectonics was wrong. Unfortunately social media didn't invent brain worms.
Plate tectonics was super controversial at one point. Would be a common one for conspiracy theorists of a certain vintage.
Social media gave a geographically distributed population of the “quirky” and their natural predators the grifters a chance to mingle on a global scale. It didn’t invent anything, but it’s a vector for the pathogen.
Depending on how much older he was than you at the time, he might genuinely have been taught in high school that this was wrong. Only in the early 1970s did that even start to gain traction. If he was taught by one of the hold-outs that esteemed a pre-tectonics geologist, it'd be pretty natural to think like that I imagine.