The first exercise you can complete is a dumbbell external rotation. Performing the exercise you want to pull the elbows back just below the shoulder, and then rotate the dumbbell to the ceiling. Make sure to use an appropriate load/weight that challenges but doesn’t impinge. 

You can also make the dumbbell external rotation more difficult by performing a slow eccentric. Remember when performing the movement to retract the scaps, squeeze the rhomboids, and rotate back. You can do this for 2 sets of 15 reps.

Another shoulder exercise using a dumbbell is a standing Powell raise. You stand with a fully extended arm across your body. Your arm is straight out and just below your chin. You want to rotate out, squeezing the rear delt and rhomboid. The dumbbell does not need to be heavy to perform this movement. 

As a baseball player, the Powell raise will help you warm up the joint to diminish the amount of friction. Friction inside a joint diminishes impulse–meaning you can’t throw as hard, fast, or as powerful. Getting rid of the friction will improve your overall power output and the force you can throw the ball.

A third exercise that is performed with a dumbbell is a drop dumbbell snatch to box. Drop a dumbbell and reach down to grab it while simultaneously performing a single leg rdl. Upon catching the dumbbell, perform a snatch and step up onto a box. The movement is similar in nature to reaching down to grab a ground ball and explode up with the torso into a throwing motion. 

The fourth and final exercise is a drop lunge with an isometric dumbbell hold overhead. The point of the isometric hold is to help the shoulder joint spend more time under pressure, coordinate with the trunk, and feel how the power from the drop lunge surges through the body. 

Do the movement on your non-dominant side. It will help create balance and more stability for greater performance on the baseball diamond.