club or recreational team?

Recreational team coaches are all volunteers. Coach can request some players, but I am not sure if all of the team members. Sometimes players can also request the coaches, but not necessarily always honored, esp. if that is a popular coach.

Do they have tryout to balance the team members?

If  a coach really wants to play to win, then it is hard to maintain 100% fairness, since players could be at very different levels.

Do not be too upset, it is very normal.  For recreational teams, it is very rare to always to stick to the same coach, unless your son is one of his favorite.