Tell him that he has graduated already. Bring him to the new school to walk around in this summer. Try to know some classmates of the new school from EK and let them play together before the school starts. :)
He know there is k1 class in the current school. He likes the current school a lot, shall I minimize the change for him and let him stay, despite the current school is not as famous as the new one.
But he never tries the new one, only verbally insist because he never had the experience of studying in the new one. Is there any experience that parents need to finally change back the school for the kids?