There is room for conflict.
On the one hand, this is a deadly assault.
On the other, once you stop your car, it is unlikely the perp can lob another one far enough, and any follow up assault is likely not to constitute deadly force. So the threat is essentially over.
This makes firing at them, or running up the hill and beating the snot out of them more like retaliation than defense. Now, the latter needs to occur, to keep them from assaulting someone else, and in some states, you are justified in using deadly force to prevent a felony or defend someone else from using deadly force - but it would be a hard argument unless you can show that you waited for them to try to drop it on someone else before dropping them.
Now shooting from a bridge is another story altogether.
Also, note that at least one iteration of the story does not sound as though the shooter turned and fired in reaction, but perhaps chased the thrower down later.
Of course, he is not around to tell his side of the story, and the “kid” has a vested interest in telling his.