The size of the object had nothing to do with him being lucky! Have you seen the size of a bullet slug recently? They're not much bigger, travel a hell of a lot slower and they kill people.
I recommend reading up on a couple of things called kinetic energy and momentum (assuming basics physics is still taught in our science deprived society).
With that kind of velocity it would have carried so much energy it would have made Dirty Harry blush, and blown his head clean off had it hit him directly. Actually it would probably have just gone straight through it without so much as batting a rocky eyelid. The effect to the kid would have been the same though.
The fact that it exploded leaving a crater in the ground gives you an idea of the energy.
So it didn't "bounce" off his head, it skimmed his head and continued the same direction it had been going... In fact at that speed it could have been the shock wave that damaged his bonce and the object didn't actually touch him.