He is working with coaches. With his goal of getting his fastball into the 90s (currently at 84-85mph) and hitting the ball over the fence more, the primary feedback has been adding explosive weight.
Here is an example of a pitcher lower body workout from Tread Athletic, who trains dozens of MLB pitchers as well as amateurs who are hoping to get to that level: https://www.youtube.com/watch?v=zkWWUHsw62M
He does like to eat, although he sometimes has difficulty finishing everything we make for him. I feel like a big part of the difficulty in getting him bigger is that he still hasn’t quite matured physically. I just don’t want to rush that, even though he feels like he needs to get there by spring to get the college offer he wants.
I have been surprised at how aggressive baseball coaches and trainers are with weight lifting programs now. I played football and baseball, and I wrestled in high school 30 years ago. I remember people telling me that my football and wrestling routines wouldn’t be good for baseball, but now those same routines (heavy bench press, squat, deadlift) are used a lot for baseball. The caveat is that baseball players have to be VERY careful not to lose their flexibility. My sons do pretty extensive stretching and hanging from a bar (3 minutes) pretty much every day.