"Shoulder Impingement Syndrome" is a garbage can term in a few ways. First, Where is the impingement, and what is causing the biomechanical abnormality in the patient? Second, general practitioners and personal trainers tend to call any shoulder pain "shoulder impingment syndrome" because it sounds fancy and gives an answer to the patient. In reality a muscle strain, or partial RTC tear is more likely (yes, RTC tears can come from repeated impingement).
"Typical" presentation of shoulder impingement results from the supraspinatus tendon (one of the rotator cuff muscles) getting "pinched" via the greater tubercle of the humerus pushing up into the acromion process (feel the bump on top of your shoulder where the seam of your shirt is). The only way this occurs is if you lift your arm up over your head repeatedly, or if you internally rotate your glenohumeral joint to near endrange (think upright row position). Maybe if he is rolling his wrist at the very end of a forehand he MIGHT be pinching the supraspinatus tendon, but I doubt it. More than likely, if he does have true mechanical pain, it results from something else.
I hope he is seeing someone more trained in the medical field than a CSCS specialist, as it is out of a personal trainers scope of practice to deal with injuries.
This is coming off stronger than anticipated, but in reality it is a detriment to patients when someone who isn't trained medically at at least the athletic trainer level try to explain injuries to patients. Maybe it is shoulder impingement, but it would take an ortho or a physical therapist examination to tell him that.
