In your video of the back view, notice how your son's glove's positon as he stops his glove's motion at 0:03 in the video. Your son's upper body is being balanced by his glove arm and his glove hand stopping at a point far out to his right - thus that where's his stride foot is planting and as a consequence, he's forced to throw across himself.
Separate the hands so his glove arm and glove point straight towards you more before he starts his motion towards you.
Now the timing of separation is something that he'll have to work out, so he finds a grove that suits him best.
And you're absolutely right about your son "filling out more."
There's also something that I noticed in the first side view and back view videos - he's not catching the ball at times. I've noticed this with adult pitchers who are either not "into" the program, or thinking about something else at the time, or just not in the mood during a session. This stuff happens.
Frustration can set in when the learning curve gets very steep, the expectations of your expectations sometimes conflicts with your son's, and other factors to numerous to mention.
So, there's a time to train and practice and then there's a time to relax and go at it some other time. I've let the pitcher tell me when he's ready to suit up. When enough time passes that tells me - he done with this, so be it, he's done - gone.
On the other hand, your son is just a teenager and this is a difficult time for some young men.