Interesting thread.
My 2018 is about the same size as yours and throws at about the same velocity. He was clocked last December at a college camp sitting right at 70. We just bought a Jugs gun, but are still figuring out how to use it, so I'm not ready to gun him yet.
My 2018 has been using the Driveline Baseball remote velocity training since the end of March. It is hard work, and it's sometimes challenging to fit the extra work in each night with baseball and homework, but it I think it will ultimately be worth it. It is a weighted ball program, but his trainer is doing a great job of getting him to the correct fitness level before introducing the weighted balls. They are big believers in the J-bands as well. I will say that since starting this program, his arm has never felt better. He often makes passing comments about how good his arm feels and how he has no arm pain after pitching. He is building a great relationship with his trainer and sends him periodic videos of his pitching mechanics to get feedback and personalized drills to work on specific tweaks to his delivery. We don't live near Driveline, so the remote option is the best solution for us. Most of the private pitching coaches he has worked with in the past are minor league players, so they are only here in the fall/winter. This program gives him a consistent person to work with year around.
I think his biggest issue right now is his growth. He is all arms and legs and he says he doesn't feel confident in what his body is doing during his delivery, like his timing is off. When you grow so quickly, everything about your delivery changes. It can take a while to grow back into your body and develop confidence again.
Good luck to your son.