2020MOM, My son (also a 2020) has strengthened his arm over the last several years with long toss. Together with a consistent pitching coach that focuses on mechanics and a plyometric style workout, he has increased his velocity. And also, he has physically matured, so some of it is definitely growing and getting stronger.
Here is the type of long toss he has done the last several years. http://www.jaegersports.com/Ar...th-and-Conditioning/