I've seen that video before by Corral, it helped a lot. The changeup is funny, it really is an individualized pitch. Some structure I'm going to try to give him is slice the ball in half (thumb and middle finger), throw the inside of the ball, keep the wrist loose, and throw with the same arm speed as the fastball.
The rest is experimentation. How loose the ball sits in the fingers, and how the index and ring finger lay is all what about what feels comfortable and is effective.