Children and teenagers tend to be more elastic than adults. Over time, our tissues stiffen and the hypermobility, like your son has, becomes more of a hypomobility as wear and tear sets in. It is in your son’s best interest that he stop subluxing his shoulder on demand and begin some physical therapy for rotator cuff strengthening. While it does not hurt him now, in the future a stretched capsule could possibly fully dislocate after which he would need to wear a shoulder brace for 4-6 weeks and follow with physical therapy. At worst, he could possibly face surgery to avoid future dislocations.