I agree with Steven’s comments especially regarding posture. While I agree about foot positioning, I wouldn’t be overly concerned given that the pitcher is throwing off of a portable mound that is small enough to limit/alter his movements.
I would also suggest keeping an eye on his glove to make sure he doesn’t get soft or sloppy. In the video, it appears to come close to floating out to the side. Have him firm it up and stabilize it somewhere in front of his torso.