The problem is going to be form and flexibility related. In addition to the supps mentioned above he needs to start doing shoulder dislocation exercises. I have attached a PDF file about them.
When doing bench a common mistake is to roll the shoulders forward. This puts the shoulder in an unnatural position and results in a strain on the joint. Next time you are training tell him to lay back on the bench as he normally would, then try and pinch his shoulder blades together and hold it that way thoughout the lift, in addition tell him to squeeze his glutes as tightly as he can. This will not only keep the shoulder in a natural alignment but forces you to keep your chest up and shoulders back. With this form you should also be able to lift more weight because the distance from lock out to chest will shorten. Practice a few times with your warm up weight until he gets the hang of it. After a couple of weeks it will be second nature. After a week or so of the shoulder dislocations and proper form the pain should subside. (Reference M. Rippetoe, Starting Strength; DC Training; Pendelay log)
Good Luck!