D.D.
You'll get plenty of info on the bruised/broken tail bone so I'm not going to chime in on that.
YOU DID THE RIGHT THING BY LETTING HIM GO TO THE PARTY. Our job as parents are to give our children opportunities and experiences to shape them into the people they will become. Yes he fell and got hurt but he also skated with a girl he likes, laughed with friends, and had that experience.YOU DID THE RIGHT THING BY LETTING HIM GO TO THE PARTY.
You don't have a crystal ball to see into the future and know what will happen in physical situations. He could have just as easily fallen down the stairs or tripped on a curb, or tripped on NOTHING (which I have been known to do). YOUR DID THE RIGHT THING BY LETTING HIM GO TO THE PARTY.