[kiwiref]

New Zealand is different to USA in many ways, but a really relevant one is the whole business of liability and the "sue 'em" mentality that we see in the American society today. I am saying this to to emphasise that officials in any game have never been sued over issues we are discussing here.
Now consider the game of rugby: very tough, no protective padding (like in football), it's all blood and guts. Concussions are fairly common, as the result of the way the game is played.
So, wether it is a game at primary school level, or a world championship game, a player who is concussed or SUSPECTED of concussion receives an AUTOMATIC three week ban, full stop. No doctors notes, no appeals to higher authority, it can be your key player, and you have the champ finals coming up, that's it.
The aim is clearly to protect the player.
How does this translate to your situation? Imagine the scenario in which that kid turns up in the last quarter, presents you with the doc's certificate. You let the kid play. The next morning, kid does not wake up. Three days later, he dies, never coming out of coma. Yes, you followed the rule, so you must feel ok?
As refs, we all come across rules that we don't like, and some that we find downright stupid. Most of the time though, we carry on enforcing these rules, as that's what we do. But from time to time, admitedly not very often, but we do come across the rule that is past silly, or stupid, or illogical, and is downright dangerous.
I would argue that if the above scenario is to really happen, you would find your *** dragged in front of the courts, and no amount of "but the rule says" and "the note was genuine" will help you feel better about what you did.