Fascinating book...read it last year. I am interested in when your son began using the weighted balls for training and if he did it on his own or actually at the facility in Seattle. It looks like none of their "onsite" training is geared for kids my son's age and it seems before doing this that supervision would be important.
He started right around the start of HS. He followed the Ranch program closely and then spent a month out at the Ranch during their summer program. He then moved to Driveline and did their "Remote" program for a couple of years. He actually went out to Seattle and trained there this summer before heading to college.
I made a deal with my son a long time ago where I would support him financially with these programs as long as he worked hard at them, which he did so it was a great experience. Not all kids are willing to put the work into doing the things that it will take to play at the next level. A lot say they do, but never end up wanting to put in the work.
I'm grateful that he is much more informed kid on arm care, proper movement patterns and the effort it takes.