There are, as expected, a lot of suggestions. Even one tool who thinks sarcasm is useful.
In the end most suggestions mean well and it may that particularly posters experience with his son.
First: Every hitter is different. Strength, bat speed, experience.... If you asked for a drill to say.... to keep hands from casting you would get more specific suggestions. In the end with out eyes on, no one can really fix your issue. From the sounds of it, he is pulling the ball. For that, let the ball get deep, drive the knob and keep his back elbow connected. Make sure he isn't casting. There is nothing wrong with trying to correct your sons swing at 9... temper your expectations but there is no reason to not teach him the correct swing path, etc. for those trying to act like your trying to get him MLB ready are ridiculous. Takes a thousand reps to create muscle memory they say. Why wait till he is ten and has bad habits before correcting them? Stress the skills and form. Teach him right so when he finally gets the size and speed and he wants to play at a higher level, he has the skill set. With out watching your son, no one can really fix but just make suggestions
We use to hit kernels of corn before a travel game at 12/u and 13/u. At 9? Maybe doesn't have the bat control. As a dad soft toss and Tee work. And make it fun! As he gets older record it So he can see his swing for himself. And when a coach tells him to not open up and says don't have your head come out... Does he even know what that means? Ask him to tell the coach what it means Be amazed how many kids shake their head yes but aren't follow the verbiage
IMO- Hope this helps