We partnered early on with the family of his best friend from HS, who is also a pitcher. The adults all run our own businesses from home or online, so we agreed the two boys could see each other. They first built a weight room in my garage, including casting their own concrete weights, and throw to each other on a schedule set by son's coach. They go to a park or a friend's land in the country to do long toss. Doesn't seem too worried about speed at this point, just building strength and working on control, although it's also possible he just doesn't talk about it.