I remember the part the article a little. Is internal rotation lack of ROM just referencing a static stretch comparison or you're mentioning? Or is their something when those pitchers throw that show an issue too. I have read about pitchers having the issue your son is having, but I have not heard of any issues in the actual pitching motion. Similar to how I talk external rotation. You can see the level of external rotation you achieve. Is there something that you can see when you look at that? I'm just curious about that whole topic.
I know personally that I have never had an internal rotation ROM issue. Of course, I'm unique in the sense that I have above average flexibility from stretching all the time.
If you feel that those areas are neglected, then you should be doing exercises to work those areas. I know I have always done exercises to work those muscle(even when I was younger, before band tubing was commonplace and before I used it). So I don't think I lack in strength in the area. But I have not heard of neglect with others guys.
I feel like the you can work those areas quite easily and they should be worked. There are a lot of exercises that can work the scap and upper back/shoulder area.
What does your son do for his back workouts? I assume he lifts.
Why do you feels those areas are neglected a lot?