strengthen his shoulders using neutral grip presses, work on rotational strength as well as the posterior shoulder as Steven said.
Invest in a device called the shoulder horn as well, it works.
I give my pitchers lots of squats and jumps to build up lower body strength and power because that hip drive is so important for a strong throw and will save the arm later.
I wouldn't bench press, and I never give my pitchers the bench to do. There are other, safer ways to increase upper body strength for a pitcher that has less negative effects on the shoulder.