You can never predict, how they will be, years down the road in high school at that age of being the "youngest." Also because, there will ALSO be many other kids, that are his same age. Late born.
I am late born. I went to Kinder at 5.
I graduated high school at 17.
I was fine. No problems.
My son, turned 5 in August. And made the cut off for entering Kindergarten. And we enrolled him into Kindergarten. Why? He was ready.
He is doing fine.
All around, academically and emotionally.
In my son's class, MOST of the other classmates, are my son's age. Only a few, are older.
Now just a tip: Kindergarten is not mandatory in many States. 5 is the entry age for Kindergarten. Typically. IF entering your child into Kindergarten when the child is 6 already, the school might enter your child into 1st grade. That is how it is at my son's school. I asked them ahead of time, about this.
Or, see if the school your son will attend, has a "Jr.K" type class/program. This is generally for late born kids. Or those who are not ready for Kindergarten.
Also, yes, if your child is TALL for his age, then, he will also be TALL in Kindergarten if starting him in school, later. And he will be the tallest perhaps. Is this a concern? Or not? You need to decide on that.
My son is also TALL for his age. But.... so are many of his other classmates. So my son does not stand out in terms of height, compared to his classmates. But again, my son is 5. And he turned 5 last August, a little after school/Kindergarten had started.
My son, is fine.
My daughter, also is later born, and she started Kindergarten at 4 and turned 5 after Kindergarten had started. AND so did a lot of her classmates.
Only a few of her classmates, were older and already 5, turning 6 while in Kindergarten.
The majority of my son's and daughter's classmates, are their same age. Not older.