Scapula displacement is usually accompanied by a diagnosis of a deficint Rhomboid, Trapezius, and Latissimus Dorsi development relative to the anterior muscles. These are the muscles most responsible for deceleration of the arm. It is highly common, especially among hard throwers at young ages that these muscles are behind the others in development. Much of that reason is from overlooking their importance in the process by the athlete.
Severe shoulder injuries, like labral tears from throwing (as opposed to labral tears from impact and falls with arm extended), are thought to be primarily caused by deficient decelerator muscles.
Core work is a loose term, but generally considered work below the lats to the hips. These are excellent exercises for pitchers, especially those looking for jumps in velocity. I'm not a PT, but wonder what benifit is being derived for the scap, from developing core?
Most "back" work is initialized with bands and then graduated to resistance with weights, although at 14yo, bands may be the better option.
The fact that it's a rotation deficit also leads to thinking that stretching of the muscles may be needed. Although, BIG ALTHOUGH, internal swelling can also be a primary cause of rotational deficit. If that were suspected, the protocol would likely be NSAID's and rest for 7 to 14 days before initiating any shoulder/back rehab work.
If it were my son, I'd be dosing 440 mg. (2 over the counter tabs) Naproxen twice daily, along with 20 minutes icing and resting for 7 days before initiating rehab. My thinking would be that I'd want to eliminate the possibility of swelling in the joint as the initial cause of the rotational deficit. At that point, I'd be all over the rehab and back strengthening to prevent the swelling and rotational deficit from recurring. Just remember that shoulders can be the toughest of the joints to help. There is little blood flow to both heal and relieve swelling, and pain is not the necessarily the major indicator of injury or severity. MRI's are often (upwards of 20%) indicators of false negatives in diagnosis. So, although it feels good, with shoulders, it's usually wisest not to try to rush them back into service based on feel. They take time to heal, you can't rush them...and that can be frustrating.
That being said, I'm not a medical professional...I'm just a Dad who's had a son with shoulder issues and studied to help him work through them (post op).
I would advise PM'ing Bulldog19 if he doesn't jump in here soon. He is formally educated in these matters.