We had the same issue with our son (literally, right down to the detail about him hating having a children's badge. He would keep turning it around to you couldn't see the big CHILD on it
) Whether or not you get any pushback, and if you do how much you get, really depends on the person you are dealing with. One year we had a lot of pushback, the next year none at all since the person we were registering with also had a tall kid. Per the last comment, I would bring any identification you have, school id, passport, etc. They may also ask your son some questions about his birthday, address, things like that.