Let's be specific. When we say arm strength, what we really mean is the ability to display power in a throwing motion using the arm. Each throw is analogous to a standing vertical jump in that both require rapid displays of power. Power is largely genetic and not very trainable. That said, on a solid strength program, you could see some small increases in power production. What we see is what we get largely with an adult quarterback's arm. Now, some of these kids may still actually be at the latter stages of puberty so there can be some developmental processes that are leading to increases in power production.
All that said, Kyle can throw that ball very hard and very fast through tight holes. He needs to work on his touch and his deep ball accuracy, which I am sure he is doing as we speak.