I was on the other side of such an event in law school. Unbeknownst to us, one third of our criminal law midterm was taken from an old exam. Virtually our whole criminal law class studied very hard for the test, as it was going to be 20% of our grade. Then, 1/4 of the class had the good fortune of running across this question and the "answer" for it during studying. Because the class was curved, the 1/4's good fortune was the 3/4's bad fortune. Since I was in the 3/4, I was pretty angry about the whole thing. Therefore the prof. declined to include the midterm in our final grade, which I thought was the correct result. But it still made me mad that he was too lazy to come up with a new question.
That being said, if the class wasn't curved I probably would not have cared.
This last question doesn't sound nice, but I mean no malice by it. Are you partially mad because you don't think he will get an "A" if he has to take another test that he hasn't seen before? It would be a perfectly reasonable response to be upset about the school reversing your son's good fortune because of what happened. I feel for you.