That teams stats show no one hitting over .350 so your reference to the .400 hitter being pulled is lacking!
Your post come across as though you feel your son was promised something and is owed something by the Summer league team. What is he owed other than a spot on the roster. Seemingly, for your son to "given" "development," some other player(s) innings would need to be reduced.
That team has 5 outfielders. One is clearly performing at a different level than the other 4.
From their stats page, it appears 3 outfielders are hitting below the Mendoza line but with nearly equal opportunities in games, innings and AB's. Total AB's for the 3 are 38, 38 and 37. There is nothing meaningfully different between the 3 from the stats page. Why should one be "given" some preference, as your continuing posting suggests should happen. Just because he is your son is not a reason to be posting negatives about the team and GM. Your son cannot be "given" anything unless playing time is taken away for other teammates. From the stats page, it appears every outfielder is being given an opportunity to perform and separate himself. At the midpoint in the season, only one did that.