On a previous post the OP mentioned the young lad attends school, meaning he will be registered at an address local to the school.
Each local "Anamai" clinic will have a full list of all addresses for their catchment area.
If someone from that address has been to the clinic before, there will be a file for that address ( our local still only has hard copy files but are looking at computerising soon). Each time a family member goes there, I guess with house book, they will then be added to the file. My name is in the file for our address, I believe my wife showed copy of my passport along with her house book. My name is in our house book as father of our daughter. This was enough for them to register me.
I have never been charged for any treatment although I did make a donation when I'd had a course of 3 chiropractic treatments.