I'm coaching t-ball this year, and the YMCA required me to take a few online training courses, one of which was on concussions and another just on general sports safety. A lot of it was geared towards players who are much older than my preschooler, but anyway...
According to the courses I took, if the injury made him fall hard and jostle his head, the coach should take him out of play and encourage the parents to take him to a doctor, just to get him checked over and rule out a concussion (the baseball itself doesn't need to hit him in the head.) If he doesn't fall, the coach might still encourage the parents to take him there (or send him there himself if parents aren't present) depending on how he reacts to the injury.
On the other hand, we've all heard of coaches who encourage players to "take one for the team," whether the player is obviously overreacting or obviously seriously hurt.