Well, the thing with the Cape Cod league is that all of these guys are so good, someone is bound to get left out of pt just based on sheer talent. (You are First team All Sec, they got a first team all american etc.)
My fall ball coach does a summer college league in the Tri State Area. Gets some good players from Rutgers, NJIT, Rider etc. Obviously he has his opening day lineup, and the 4-5 guys that start every game because they are that good. But he also shifts around to get every player some starts. For example, he may have his stud third baseman play first or DH in a game, and give a younger guy a start, while keeping the stud's bat in the lineup. Real standup guy. Great coach.
I think it varies on a league level likely. Also, I am new to the site, so I don't know your/your son's history, but is there something that maybe the coach sees in other player than he does in your son? Maybe something generally unnoticed like speed, or aggressiveness? Don't take it the wrong way, I do believe there are some coaches would show clear preference to certain players, but was just wondering if there is another side to it.
How are the other 3 outfielders playing time on their college programs as opposed to your sons?