My son had a early Sept birthday. Most kids in the class behind him were a year older. You guessed it they all started a year later.
One year we had him play with his age instead of his grade in the Fall. And while it was fun for him, it really did not help that much. I would have rather he played with his grade, looking back and face tougher competition. Played with his grade or up the rest of his HS/Travel Career.
So much can happen from 8th grade to graduation. Plus I cannot imagine making a kid repeat 8th grade. If you are concerned about his age, then do it before he starts school or a Post Grad.