There's a handful of Back to the Future theories floating around the internet, and one, which posits that George McFly eventually knew "Calvin" (a.k.a Marty) was his real son, changes everything fans have always thought. The 1985 sci-fi comedy follows Marty's (Michael J. Fox) journey into the past. Of course, the world now knows how seemingly harmless time travel can have damning consequences if done incorrectly. Marty (who famously gains the alias of "Calvin Klein") travels back to 1955, meeting teenage versions of his mother and father. Even so, despite pretending to be someone else during his Back to the Future time travel journey, Marty's father, George, may very well discover that Calvin from his youth was actually his son later in life.
The story of Michael J. Fox's Marty in Back to the Future opens with him as a normal California teen. The year is 1985 and he's concerned with typical high school-aged things, like his girlfriend, his musical aspirations, and becoming his own person that's separate and different from his parents. Then, "Doc" Brown (Christopher Lloyd) shows up with a DeLorean that can defy the rules of time, and the normalcy of Marty's life goes out the window. He's transported back to the 1955 version of his town and many of its citizens, including his own parents.
There's a Back to the Future fan theory on Reddit that posits the idea that "by 1985 George McFly was able to figure out Marty was his own son from the future." The post's author points out that Marty does a handful of things during his Back to the Future time-traveling escapade that would turn out to be eerily accurate or sort of "before their time" (such as predicting the town's future mayor and fashioning his own skateboard). In addition to these things, the Redditor highlights other societal developments that would likely catch George's attention over the years. For instance, the Calvin Klein clothing company would eventually be founded, musical artist Chuck Berry would release his song "Johnny B. Goode" (which Marty does a now-famous cover of in Back to the Future's latter half), and much more. All this indicates that George would have been able to easily figure out that the man he knew as Calvin Klein was actually his son from the future.
There's an extensive list of things that George McFly (most likely) would've noticed as his life and general time progressed. The fan theory also brings up that George is an avid sci-fi fan in Back to the Future, which would make time travel a much easier concept for his character to believe and accept. In addition, the Reddit post also links to what is supposedly an early draft of Back to the Future's script. This version ends with George looking at a newspaper clipping featuring a picture of Marty back in 1955, shaking his head, and saying aloud, "Nah. Couldn't be." The deeper one follows the threads of this fan theory about George realizing Calvin was his son, Marty, all along, the more evidence there seems to be in favor of its veracity.
Even so, despite the plausibility of this Back to the Future hypothesis, it's hard to believe that it's true in the context of the film that was actually released. A key part of the movie is that those who are part of the 1955 reality aren't aware of Marty and Doc's Back to the Future time travel. Marty's parents, in particular, are so unaware of who he is that his own mother kisses him, only to discover that she feels as though she's kissing her brother. Still, if the film's events happened in real life, it's next to impossible that George wouldn't eventually suspect (if even for a fleeting moment) that all of the recurring coincidences regarding Calvin and his son had more to them than just pure chance.