Tough to comment with out seeing his swing. And I am sure you will get a couple dozen daddy experts just like me making all kinds of adjustments and changes. If he is pulling a lot of balls, sounds like he is not letting the ball get deep. continue to soft pitch to him and explain and work very hard on allowing the ball to get much deeper in the strike zone. That is something ANY one should do hitting a ball. But with him pulling the ball, it would help to start having him get to understand the concept.
Keep soft tossing to him though. Keep working that hand eye contact. Have him drive the knob. If your unsure of how to correct his swing, there are a ton of hitting clinics that are perfect for a 9 year old. But as a dad, stay positive. If you get frustrated, so will your 9 year old. Believe me. I have learned a few things from mistakes as a father coaching his kid.