Thanks for your comments Coach Paul, Ted, and JD!
Honestly, this board specifically has a lot to do with his improvement so far over the years--both through comments and one-on-one work with members (and I thank you for that).
As far as strength and conditioning, for about the last 6 months, he's been working with a program developed by LankyLefty (Ben), which has been fantastic. Ben has really helped his strength a ton...and helped him understand what a good nutrition and strength plan looks like.
Mechanics-wise, he's been working with Baseballthinktank's and Driveline's drills, almost exclusively. Lantz has been a great mentor, and really helped him understand the body movements/separation that go into the kinetic chain--even working with him one on one a few times. Kyle's drills have been extremely helpful for arm action (wrist weights/weighted balls), and Kyle's Hacking the Kinetic Chain book and materials is just ridiculously good. Additionally, I've tinkered with some added med ball drills that he's been doing to improve overall explosiveness.
Regarding the arm conditioning/sore arm question, a lot of Kyle's stuff focuses on that, as well as Ben's (working deceleration muscles, arm care protocols, etc.). Hopefully things will go well this season! [He's definitely throwing hard--I'm getting close to hanging up my catching career.]