How old is he? Height and weight?
There are a number of things that could be causing the shoulder stiffness. Not fully using the hips and creating max hip and shoulder separation could result in him trying to make up for it with the arm. Throwing across the body could cause the humerus to operate at the limit of shoulder's range of motion. A timing problem causing early shoulder rotation could cause the arm to play "catch up". Growth spurts can cause various aches and pains. Lack of proper conditioning?
Stepping straight towards home plate could improve his use of his hips to achieve more hip and shoulder separation. And that could result in an increase in velocity.
In my opinion, what you do to create movement is no different for a submariner. Regardless of arm slot, you still need to crack the whip although the wrist is always straight at ball release regardless of the pitch thrown. What does seem to increase movement is to get the release point out front.
Sure! There is a forum on this site just for that. Post your video on YouTube or other hosting site and then post a link to it in the Video Ananysis forum (under the Mechanics forum).