As said, not much you can do to him without a confession that he put the rock there with intent to harm someone (he could say he put it there, starting an erosion control dam or something).
It's too bad you have a permanent injury from this, I'm sorry about that, but sledding is inherently dangerous and I'm glad I haven't sustained any injuries that I couldn't fully recover from. My wife isn't quite as lucky. She broke her tail bone a number of years ago and it hasn't quit hurting.
Just do some fronteir justice on the guy and DON'T post it here! Self satisfaction will be enough.
Now that enough years have passed, little story...a buddy and I were riding thru town up in N Wisconsin, ON THE SNOWMOBILE ROUTE, SLOWLY. He went around a corner in fornt of me and a homeowner had strung a wire fromt he phone pole to the nearest tree in his yard, at head height. It caught my buddy, just below the neck, across his chest and whipped him off the sled, only bad bruises.
After cutting down the wire and seeing it back up again a week later, we went over there about 3am, 2 chainsaws warmed up, idling in the back of an anonymous pickup truck (another friend who wasn't from around there, no tags
). Hopped out and cut down the 2 trees nearest the phone pole. By the time his light came on, the trees were hitting the ground and we split!
Left a nice row of 40' tall spruces in front of his house with a large gap in them.
We felt better!