Proper warm-up, watch the pitch counts, don't over throw. Don't worry about the slider. If he has got a good one at that age, use it. Rotator cuff exercises; i.e. lateral raises from front, sides, and back.
Common Myths: take time off "Pitching is the only activity that the so-called guru's expect you to get better by doing less", -Leo Mazzone
"Injuries are not from one type of pitch. They are caused by improper warm-up, over throwing, and too high of pitch counts" -Leo Mazzone
"Out pitchers throw every day. Not pitch, but throw. Also, we don't take time off. We throw all year round. You won't take time off of your exercise program and expect to get better." -Leo Mazzone