Pitching coaches that coach technique(s) come in various instructional levels and exposures. Based on a point or points in time that they themselves were coached and applied said same. So, what a coach will point out, has to do with how he/she found their instruction to be - successful or not.
Now here's the thing about coaching youngsters in the formation and maturing years of their life - they'll grow, develop, and mature at different rates and at different times in their life they'll through all kinds of hormone and other adjustments. So, I wouldn't be a bit surprised to find your son doing something completely different, every two or three years, as the years pile on,
This "momentum" that's quoted by many on this web site is true and factual, to a large extent. However, try to apply that, along with other suggestions, in the real world that amateurs have to deal with can be a hard sell sometimes. Uneven and deteriorated pitcher mounds can be a nightmare for a youngster trying to emulate coaching, then pile on just for good measure, disappointments from those who are paying for private coaching, and a host of other assorted agendas.
I'm not knocking good advice, nor am I finding fault with ... do this... do that. I do have a problem(s) with coaching via this media, trying to tell you ..how to...when to.. and other pointers for you and your boy to follow, without understanding the ramifications of taking advice from this web site and literally smacking the paid coach/instructor with it, as if YOU KINOW WHAT YOU'RE TALKING ABOUT. If you intend on bringing this issue up, just be ready to acquire the label that you're about to wear and the impact on your son's playing experience with other coaches during his summer league.
Look, he's my purpose for posting the above - . your son's physical ability, his attention span, his paying environment and the conditions that he deals with field integrity/the lack there of, all require hands on - being their. We're not. Add to this is your agenda(s), you attitudes and expectations, what you expect from opening your checkbook, your observations of others who do things differently and so on. Have you asked this coach why he does what he does? Where did he learn this technique(s), etc.? I admire your concern and wishing the best for your boy. Just be very careful how you go about "managing" your boy. Parents can get the title "control freaks" real quick without realizing it and that label can go well beyond the intended mindset off the playing field.