Schools out for some...er?

Hello everybody, I would really appreciate any input on easing our  six year old son  into an enforced school change please.

He is currently in year 1 of an independent school with SEN support. WIth opposing views between me and his mother, he has never been assessed to the point  diagnosis but it is generally accepted thathe is on the spectrum. However, We have had a clear heads up that the school does not see him making the independence  that he will need in year 3. 

So, trying to get ahead of the inevitable, We have a sampler day arranged at another school. how do engage a bright but sensitive boy in the idea of trying another school for day please.?