When you are young, especially, or when you are FREAK STATUS STRONG, it is very possible to "over train/strain" your joints/tendons. A lot of people who use LARGE amounts of AAS (etc) end up with over recovered muscles and under-recovered joints and tendons. All leading to injuries.
He just has to change some things for a while...
well Im not freak status strong, lol. Change things? well yeah Im planning to stop the heavy triples and sets of 5 for bench and squats for a while and keep away from upper body till this shoulder heals fully...