He plays several positions. He also plays football in the fall and wrestles in the winter. Most programs aim at muscle growth, without much understanding of muscles growing - as your son's still are, of course, at age 15. One wrong move could be the start of a bad elbow, bad back, etc. It's good your son mixes sports.
Every sport works muscles differently.
He'll develop agility in baseball, speed in football and strength in wrestling. That will make him a well-rounded athlete. Weight training isn't unsafe, but there are unsafe ways to lift. Your son should be properly trained and supervised and focus on high numbers of reps and low weights. A good routine is half an hour of weight training, no more than three times a week.