I think the son was switched due to his clone having the burned face. They mention the fire and I believe that the grandmother died in a fire the year before. I think that the son started that fire doing his magic trick and was burned as a result. I think that’s when the switch took place. If you remember they said they’d been waiting for them to come back. The son spent damn near the entire movie trying to do the trick and couldn’t. At one point the daughter even mentioned how he couldn’t do it anymore. Also the son didn’t really have too many lines in the movie except for some basic shyt. And he kept saying bad words. Which had me thinking maybe he’s still learning how to talk. His clone who was the original him was acting like an animal almost basically becuz he was going crazy from living underground. At the end of the movie him and Lupita gave each other that look like they both knew what time it was
Anyway that’s how I interpreted it. I’m prolly wrong tho