My DH was a kids football coach for several years, and up to about the age of 11 did try to ensure that all kids got a reasonable go at playing.
However, from 11 upwards the leagues become competitive, with placings and trophies for the team who won the league. So he did generally pick the stronger players, trying to ensure weaker players had a game every few weeks, or prioritising them for friendlies. He was not allowed to take a player to the game and not play them at all, so if picked, the player would definitely get game time.
He did try to make clear to children and parents that players were picked on a mixture of ability, commitment, attitude and behaviour, but ultimately some children just were out of their depth at the level the team were playing at. All children were welcome at training no matter what their ability, but not all would be picked for the team. I
It's really hard on your DS, and I do feel for him, but I do also think it doesn't necessarily mean that the coach has a place in Hell waiting for him!
It's probably worth a chat with the coach to find out the basis on which the team is picked. It may also be worth looking around to find a club, maybe in a lower pool, where your son may be playing more at his level. The coach may even be able to help with this.