This is exactly what we did. He was born in June, literally the middle of the year so we kept him back and he is the oldest second grader in his class. Didn't get an IEP, we knew he had ADHD and it has an official diagnosis and we intentionally kept him out of public schools because both my wife and I have ADHD and knew he wouldn't do well in a traditional desk setting. He goes to a Regio inspired school on a farm so he gets to spent most of every day even in winter outside which has been a game changer for his development.