Start here:
ericcressey.com
drivelinebaseball.com
Jeagersports.com
theathleticpitcher.com
I suggest you add https://armorypitching.com/ to your research for velo. I see that you are from GA. Randy Sullivan is friend, former college catcher & academic all-American (Citadel). Owns a rehab business and a pitching academy near Tampa FL. I don't want to get long winded here. Just check it out. 90 something pitchers over 90mph. Pitchers solving pain issues. It all starts with a pitcher assessment (physical and video). He txt'd me a week or so ago. He's out of 90mph black shirts in size medium. Has plenty of XL though. So, it's not all about the size.
Here is a shameless plug. He's hosting a webinar tonight at "Arm Care Summit" with Ron Wolforth, Flint Wallace (Texas Baseball Ranch), Kyle Body (Driveline Baseball), and others top velo coaches. $30. The topic is "Lower Body". Tonight 8/28 at 8pm EST. Some of the best minds in pitching. They don't have it all figured out, but they are working on it. https://armorypitching.com/ac3
Another shameless plug. If you can't get to FL and can get to Charlotte NC. Come see me at http://www.throwharderfaster.com/ I'm not Randy, but I did have a 2015 student hit 89 this July at East Cobb (both with the 17u and 16u groups) and he was 74mph this past summer/fall. He hadn't played baseball in 3 years. It took 11 months. I had coaches, dads, and even my son say "you are wasting your time". He'll never throw 90. Even some dad said, "I'll bet you $1000 he never hits 90." Well, I didn't bet, and I"ll never see that money. But he'll hit 90. He's sitting 87-88 and was barely 80 this spring. 83-84 by May. 86-87 June. It took a physical assessment, video analysis, functional strength training. (had to get him out of body building style weight lifting). we started with his arm action, had to address some pain he had, and worked backward up-the-chain, to his lower body. I have to admit. The jump from mid-80's to 89 wasn't all me. He learned to control his hips and another coach saw him collapsing his back leg prematurely. And here is the funny part. His command got better. Painfree for months now. Best part was seeing his dad's face w/him on the mound after having not played since 8th grade. Saddest day of his dad's baseball life was when his son quit baseball after 8th grade. Dad's not so sad anymore. Mom's happy too. The player is excited to pitch and learn and develop. That's why we coach.
He still needs to learn to pitch, manage himself and the game (Me vs. Me vs. You) But at least he has a chance to chase the dream.
Randy, Ron, Kyle and Flint, Eric and others have scores of stories like that. I bet they can help. Best of luck to your son 1515. Sorry this post was so long.