I, too, am confused. Do you mean a form that allows the coach to get medical treatment for your son if you're not available? Or do you mean a form that allows the coach to get information from the doctor that states he's medically fit enough to play the sport? I think both of those are provided to you by the team. Part of my confusion is that the forms should have been included in the printed information given to you when you asked to sign up your son for the team.
The doctor may have his own form that will state the he's fit to play.
You can write your own note that gives the coach the right to obtain medical treatment for your son in the event that you're not available. That is all that it has to say. Or you can narrow the options to for example only when the injury is related to the times that your son is under the supervision of the coach.