Rotator cuff exercises for a 9 year old?
I know your son wants to pitch now and that's cool, but almost every guy I played ball with who started pitching that young, had an elbow or shoulder injury before they were out of high school. If you want a long term (high school, college and beyond) career for your son as a pitcher, the best advice I can give is to keep him off the mound until he gets into high school.
The best advice for strengthening a throwing arm is to long toss. I don't know what impact it will have on him at the age of 9, but that's how you strengthen your arm.
When he is ready to pitch, teach him a changeup. A good changeup is more effective than a good breaking ball. Randy Johnson is a dominating pitcher, but over the long term Pedro Martinez and Greg Maddux have been far more effective. Why? They both have superior changeups. Plus, it's far easier on the arm.
I had Tommy John surgery in '99 due to years of throwing curveballs and sliders with no changeup to go with it. This was my first year back after my surgery and I had my best season in years. In our playoff game, I held the best team in the league to 2 earned runs. I threw 110 pitches: 64 fastballs, 34 changeups, 12 breaking balls. I wish I had developed a change when I was in high school.