I’ve worked with pitchers who have had a similar problem. Diagnosis by a qualified Medical professional is essential. Not a Nurse Practitioner, not a physician’s assistant, not an RN or LPN, but a medical professional who deals with the muscle diagnosis for your son’s age group.
More than likely, the muscles of the trapezius, the Terse minor/major and the levator scapulae might be a concern, but then again, a medical professional can give you a better definition.
You mentioned something about a time frame for your son – don’t. There is not time frame for your son to do or not to do anything, with this condition that has yet to be defined, exactly. Now you’re talking about a quality of life issue later on in his mature years.
In all cases that I’ve had experience with shoulder weakness, it has been associated with a condition at birth, regardless how small, and an issue that many athletes don’t discover until the demands of their sport cause awareness.