My daughter 5.9 (no obvious signs of giftedness, reading at a second grade level and thinks out of the box) just finished public kindergarten from a Montessori preschool. Her preschool teacher told me she was a year ahead, so I assume that she was receiving some kindergarten instruction while in class 12 hours a week.
Though she wasn't challenged a LOT in public Kindergarten, they are all at such dif levels in Kindergarten that she was able to read her own books in class. She didn't get much instruction in reading, but wasn't forced to read what the other kids were reading. (She was the youngest in the class by the way ....cutoff in Jan...and a lot of red-shirted kids).
She WAS taught how to form her letters and numbers and loved learning songs. Some of the Math was new to her (even though it was taught VERY slow). She was in no hurry, and she enjoyed many of the social aspects of school. She loved the building materials and the bars. I also think that the teacher was a little more patient with her "organizational skills" (lunch card, snack card and transition times) because she was the youngest and smallest in the class.
So, my daughter may not be AS ahead as your son, BUT being the youngest in the class was a benefit to her. AND don't forget to look into how many kids are redshirted in your district before making your decision. there were SIX in DD5's class of 16 this year!
