First off let me say this: I know you're mad, but when you typed that he punched your car at 75% strength, I laughed for about an hour. To imagine someone rearing back with their fist and letting loose at 3/4 strength EXACTLY is just humorous to me.

Now to the issue: No matter how flat you make a pancake, there's still two sides. In this case, you meant to just play a harmless joke on the kid, but in his eyes he was doing you a favor by going in and he very likely (in his stoned mind) was hurt by what he perceived as a mean-spirited joke at his expense. He reacted poorly by physically attacking your car. Violence is always a last resort to problem solving, and usually ends up as a failure.

You have to decide if you want to remain friends with him. If this incident was a deal breaker, then so be it that's your choice. But if you value his friendship over his actions, then you need to go find him and you both need to settle this matter, preferably with a shared joint ALONE. Talk out your problems and really you both need to apologize to each other and bury the hatchet.

Just my thoughts.