Ok. I can believe this. But, there are a lot of questions about it. How does the double get to the top to make the change? How was the double able to speak? There is no evidence that suggest Jason had any type of treatment or went thru a period where he didn't talk. Jason is the way he is because he is mixed with tether and regular. But hey, I wouldn't totally dismiss this theory. That's the great thing about the movie.
Jason wasn't a double
He's just tethered and original mixed so he shares the traits
He knows his mom is the tethered at the end.