Something tells me you've never thrown a baseball in your life. Buy a mitt and throw a few around with your son.
The most correct advice given belongs to NLBwell. I'll just rephrase it....bend your elbow. When you begin the racquet throw, you UNbend your elbow. What you're doing is rotating your shoulders instead of throwing your arm. That's why you jacked up your shoulder in the first place.
I honestly believe some of you guys grew up without a proper childhood. I do see myself as somewhat of a jock, esp compared to all the nonathletic people out there. I think throwing a baseball, catching a frisbee, playing dodgeball,....etc are part of a proper childhood for boys.