If the book is two grade levels above his reading, he's probably not going to understand the book very well. (It also sounds like he should be taking remedial reading, because reading comprehension becomes WAY more important in middle school and high school.)

Judging from the LA Supervisor's note, he doesn't realize that Charlie is going to have serious problems comprehending the story, or even understanding that, while the N-word is used throughout the book, it's STILL not acceptable to use it. He sounds like he thinks Charlie is an ordinary bright seventh-grader who doesn't want to do the summer assignment and whose mother is being a little overprotective.

I think you need to get through to this man that Charlie is NOT going to be able to understand this story the way a kid who doesn't have his problems would--that it's not a question of talking the story over with Charlie, but of Charlie having problems understanding why people think and say and do the things they do NOW, never mind why they thought, said and did things almost a century ago. And you really need to spell it out in those terms. Give examples of Charlie having severe problems comprehending appropriate behavior. Because judging from that note, the teacher does not understand just how little Charlie will grasp.