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.

oh he understood what plate tectonics was proposing very well, he just completely rejected it in favour of many distributed prehistoric oceans and a lot (a LOT) of erosion. It was weirdly congruent to phantom time theory.