A.
Don't approach it with the idea that the coach will be fired, and don't go into this seeking revenge. Look at it more as an opportunity to have policies put in place that will prevent this from happening again. You might suggest that there is a mandatory report to be filed whenever a student receives a head injury or loses consciousness, that parents be called immediately, etc. You DEFINITELY should have been notified and not had it left up to your son to tell you.
(My son came home from school a few weeks ago with a large bump on his head and I was upset I wasn't notified about it - he didn't have a concussion, but I feel any head injury warrants further observation. I approached the teacher who had helped him after his fall, and it turns out she didn't know about the bump on the head because he only complained about wrist pain. :) )