D.B.
As Mel R. says, the first thing you do is have your child evaluated by the pediatrician. Your child sustained a head injury, and it doesn't matter how it occurred or whose fault it was - he gets checked out immediately.
Only after that should you find out if an incident report was filed by the play area supervisor. If not, why not? Again, it doesn't matter (at least at the outset) that the perpetrator was her son. (And yes, you must proceed with the assumption that you only know a part of the story - that from your son, who had a head injury.)
Supervisors don't ask kids if they "want" to go to the nurse. Not if they are 9, not if they are 16. There's a head injury, they get seen.
In our school, there is a special insurance policy to cover expenses incurred by families for injuries and accidents on school property. My son was covered by such a policy for an injury that occurred when he tripped over a raised structure of some sort. (No other kids were involved, but my son ended up on crutches and with a sore back.) So your doctor should fill out a report and send it to the school - not just for your expense of copayment and any tests, but to officially notify the school that something happened on school property that required medical attention. That filing usually gets their attention very quickly.
Before you make accusations that the other mother/supervisor didn't provide a "referral" (I'm not sure what that term means), you have to find out if that's what happened. You don't say how you received that information. Just from your son? His info may not be accurate - even if he's an honest kid, he's only 9. He might not get the info correct. Or he might be shielding you so as not to worry you, or he might be trying to defend his decision to lie low and not seek attention.
So yes, you should contact the school, but absolutely not until you seek medical attention for your son. Then talk to the doctor (have your son in the waiting room) about what you should do next. (Doctors usually know the procedures at local schools.) I wouldn't go in full bore with an attorney, but if you have a family lawyer, it wouldn't hurt to get a quick consultation on what you should do and what you should NOT say. That may be important.
I think the main thing to do when you talk to the school is to ASK first, and say very little: What is the policy for injuries sustained on the playground or elsewhere on school grounds? What is the responsibility of the supervisor? What is the policy if a child declines to go to the nurse? How are children monitored after a head injury? Don't answer - just write stuff down, asking questions only for clarification. You have plenty of time to get ticked off - but you have to get info first.