It really depends on how you fall. The ones you mentioned are the most common sites because of the way we guard ourselves during a fall.
Scrapes elsewhere happen when we don't have time to or are unable to react. Such as if we get in a fight and get floored (except on gravel) or just fall backwards. Things like a faceplant are super common for kids.
Point in case, that is not the underside or all that close to his armpit. That's somewhere on his triceps. Try assuming the same pose with your hand, touching or sticking something on that spot, and see where it ends up when you return to a neutral pose, and you'll see it's exactly where you would scrape yourself if you fall backwards or on your side.