I'll add one more thing.
There's little he can do about it now. If he wanted to grade the midterm differently, he'd have to track down all the students now on summer break or graduated, figure out which ones made this error, regrade, and then submit grade changes for all those students. Even if he had the exams to look on file, it's unlikely to be something they want to do after classes are done and final grades submitted. If it was the final exam it would be a different story.
You're not approaching him to fix the error, you would have done that when the midterm grades came out and you saw your exam. You're doing it because you were a few points short of an A. That changes your argument and his motivation, especially if it means regrading 6 week old exams. Unless there's more to the story, I see the conversation going something like, "I wish you had brought this to my attention in March, but there's nothing that I can do about it now. Have a good summer. (Followed by screeching tires as his 2002 Z3M convertible tears out of the faculty lot for parts unknown)
--Il Destriero