E-k-1 teacher here. Two kids--sent to co-op pre school. I taught first grade for 4 years before having them and I had some preconceived notions about what type of experience I wanted them to have, being a non traditional learner myself.
So I opted for a co-op non hurry-up and learn for my kids. My son was a spatial, non-early reader, out to lunch, observer, sponge type (he sucked up every thing and nothing came out) of kid. My daughter was a linear, early everything, up front, highly sensitive type of kid.
Both had birthdays mid year, so when it came to kindergarten, my son was 5.5 years old. I went to talk to the principle and teacher. When I was describing my son they suggested the 'special' k-class. I looked at it and the curriculum and knew my son would be bored out of his mind. So I put him into a Montessori school. My daughter followed. My son stayed until 5th grade, my daughter until 3rd. Best decision I made.
So, I suggest you look at the curriculum for the first year. One year will not particularly label him. He may do so well he will not need it after the year. Also will his brother go too? That will offset the labeling. Ask if he has to stay with the group through k-6.