I would look into another couple of factors.
1. How is his behavior in a group setting? Will he be able to handle expectations in the classroom, moving from a small K class (5 kids) to a large 1st grade class (I don't know how things are in your area, but our public schools routinely have 25 kids or so in each 1st grade class).
2. How is his reading, math, etc compared with other kids who will be his classmates? Again, in our area, a lot of kids already read and do simple math when they enter K.
3. What does the new school suggest? I would talk to them and let them know the bullet points that you listed, and see what they say.
