Thanks for your input.
I have to agree, that when I imagined that one day he goes to K and the next day into 1st, it just did not seem right. I would prefer slower transition, and you gave me some great ideas. I am not worried about the academics (as he is still well ahead even of the first grade), or misbehaving. My main focus in this is the emotional part. DS likes things to be consistent, fair and predictable. Although, I think once he will see that the 1st grade is just more grown up in the whole learning approach he will appreciate it.