we're putting ydi because it seems fairly evident op wasn't wearing a helmet.

It's the severity of the concussion that is making people jump to that conclusion. The fact that his brother smacking him on the head caused him to pass out implies that it is pretty bad, nless the brother hit him ludicrously hard. You have to hit something really hard to make a helmet that useless and it's pretty hard to do that simply by falling backwards. When hockey players get a concussion it is usually because they were hit by a really fast moving, hard, puck, or were slammed into by large, reasonably fast moving people, on ice. They generally don't get them simply because they happened to fall over and hit their head on the ice.

Football players wind up with concussions because they are being continuously slammed into by other people. They generally don't just fall over, hit their head on a hard patch of ground, and wind up with a concussion. As I told someone up there who said the same thing in regards to hockey players, it is the severity of the concussion that is making people assume the OP did not have a helmet on. It almost takes talent to smack into something hard enough to render your helmet pretty much useless, simply by falling over backwards.

Unless OP is a really big klutz and/or severely accident prone. I ripped a ligament and had micro tears in the arch of my right foot, as well as severely sprained a tendon on one side of the ankle and a ligament on the other. How did I manage this? Slipping on a rock during marching band practice. My foot bent back too much, like where your toes go back towards your leg, causing the tissues and ligament in my arch to rip due to the extreme tension and the ligament and tendon in my ankle to severely sprain when it twisted as I stumbled to stay standing and keep moving rather than falling. The point? OP may have just hit a sensitive spot in his head or a weak spot in his helmet. Maybe he hit at an awkward angle or something.