Z.B.
You have to stop talking about it. September is more than 3 months away, and that is an eternity to him.
There's really no reason to bring it up again until about a week or so before school starts. Most schools will have a "Meet the Teacher's Night" before school begins, where child and parents have the opportunity to visit the classroom and meet the teacher and get a feel for things.
You have lots of time. Relax! Give him some time and try again later. He'll be fine.