DS is starting the transitioning next week.
On Monday he will meet just the first grade teacher (when kids are in specials), so she can show him around the classroom and explain how they do things. On Tuesday he will spend some more time in First, and afternoon we will do a goodbye party in the K. DS will spend more time in First on Wednesday, Thursday almost full day and Friday he will go with the first grade on a field trip. The following week he should be full time transitioned in first.
I think this should work.
Thank you all for helping me with this.