When you think about a pitcher short-arming the ball, only worry about the moment when the shoulders rotate toward the plate (the most violent part of the motion) and whether the elbow is lower than the shoulder. That will cause the undue strain on the shoulder. Without seeing the pitcher, it's tough to correct what's going on. Is the pitcher able to throw strikes with the occasional short arm? Is there any pain caused by it? Is there a drop in velcity?

A change in arm angle can sometimes come from a change in whole body tempo. For example, if a pitcher moves his body toward the target at a faster tempo and gains momentum, they can take a full arm path. When the body speed slows, sometimes a pitcher compensates by taking a shorter arm path (short-arming). Take a look if you see the overall tempo change when throwing like that.

Hope that helps.

Pat

I agree with Pat, especially about dropping the elbow, another thing I would for and is common with young pitchers and it goes along with Pat ideas on full arm path, is they generally want to rush everything, getting out to fast on the stride foot before their in the highcock position, then when they try to catch up they drop thier elbow, but like Pat said it's hard to tell unless you can see the player.

Ripken teaches a drill where you stand behind the pitcher and have him get his arm in the correct top position: palm toward 2nd, elbow above shoulder (sometimes called the swan position). Flip a ball to the pitcher and have him toss it back to you or have him throw it to a partner. This ensures/ conditions the pitcher to get his arm in the right position.

Carl, I just posted this as an answer to another question, hope you find it useful.

This drill can be done indoors or out...
Take a small towel, (dish towel for instance) and tie a knot in one end. Get yourself in proper throwing position (athletic stance, feet shoulder width apart) point your glove elbow to your imaginary target and, in slow motion, go through your delivery holding the loose end of the towel. Where the knot would land, place a bucket or similar object. Repeat the above process in regular speed concentrating on hitting the bucket top center each time.

For my son, we have measured his full extension and marked it with painters tape in our basement. We also marked a straight line so he knows the correct path to follow. I would recommend about 20-30 of these per day to teach muscle memory without wearing the arm out.