Fuck it I say it too.So because You say it? Lmao
I think the movie went out of its way to establish Jason was different - he's the baby, he's a c section baby so the mom had kind of a stronger connection with him than the daughter, he's into stuff nobody cares about, he wears that mask for no ass reason and he's kind of a loner...
I read it as he has somewhat of a 6th sense like how all the stark kids in GoT have some warg abilities, with bran obviously being the strongest. He was connected to his tether without actually knowing what/who it was...like a twin thing in a way. That's why he was building the tunnel and why he had that trick with the fire. Once he became acquainted with his tether and learned its mannerisms and the mimicry that's when he started noticing something was off with his mom cuz he understood how tethers worked...fast forward to the end that's how he knew his mom was actually a tether and not a human.
The twist of Jason knowing who his mom really is doesnt hit the same if he also got switched.