This question is so open ended that it's difficult to nail down an answer. I am not a mechanic's coach, never have been and have no intention of trying to impress someone that I'm knowledgeable on the subject(s).
Strictly from an observation standpoint, I would say this:
-Age along with certain physical attributes goes a long way here. A 12 year old just learning things, along with a 20 year old just starting out have common ground on the subject. By that I mean, certain muscle structures and mental focus, open mindedness, or not, goes a long way in being receptive to coaching.
-Physical makeup (physique) also plays a big role. A slim yet athletic preteen can be just as coachable as a slim yet athletic mature individual - but, this changes drastically as the proportions of a person goes into noticeable height/girth, and even certain motor skills disproportion. For example, coaching a stocky, wide girth, overweight individual is extremely difficult regardless of how proper the coach is, or should be.
-Want and need to allow the body to choreograph all the moves that a pitcher will go through - suitable to that individual, is THEE biggest challenge that I see. Again, I'm not a mechanic's coach, but I've witness some ill-faded attempts to coach someone with "do-this/do-that" totally foreign to the physical and mental ability of the trainee.
I could go deeper into the subject, but I think you get the idea. I assume this is in relation to other posts relative to your son's progression and what's best in that regard.
I would suggest sitting down with a coach who coaches pitchers exclusively and have an appointment to talk, listen, ask questions and get your feel of the topic and questions that you want asked. In that regard:
-Ask this coach to appraise your son's physique, how healthy he looks, his attention span as you listen to this coach(s).
-Ask what priorities does he (coach) focus on first, given a "first impression" of your son. Why?
- Ask what first impressions are he looking for doing certain activities - playing catch, video of games played, etc.
Lastly, I would caution going into a meeting with preconceived ideas and assumptions. Going into a meeting is just as much a learning experience as it is being impressed by someone's knowledge. That being said, I've seen some sit in a meeting with a potential coach and literally take over that meeting with trying to impress - even challenge, the coach that they're meeting with.
I apologize for posting questions rather than explicit answers. I trust my remarks are helpful in some way.