Not sure you can step forward twice with your stride leg. Stepping to the side is an accepted rules violation, but I think even the most lenient umpire would object to a forward step with the free foot to initiate a delivery.
…In his actual delivery of the ball to the batter, he make take one step backward and one step forward with his free foot. Backward has come to be interpreted as back or to the side. Side has come to be interpreted as parallel to the pitcher’s plate. Your first step is decidedly forward and an illegal pitch.
Due to his stride direction, the first thing I’d do is move him to the glove side of the rubber so that he’s striding towards the centerline of the rubber instead of away from it. This will help avoid posture issues at release. Also, this is really a “freebie” because it doesn’t require any change to how he throws the ball.
Next, I’d have him work on moving toward home plate sooner. Right now, he doesn’t start forward until after peak of knee lift. Moving forward sooner will generate more momentum and will make him quicker to the plate.
Last, he needs to improve his glove arm. He needs to extend it more to mirror the throwing arm (opposite & equal). This helps both balance and timing (keeping the shoulders closed longer). Then he needs to stabilize the glove in front of his torso instead out to the side.