State or private schooling for child with mild/moderate autism?

My 3 1/2 year old son has recently been diagnosed as being on the autism spectrum. He's mild in some areas and moderate in others. He was due to go to our local state school nursery but the SENCO there was less then helpful and made it quite clear that they didn't really want him there so we've decided to keep him on at his preschool for another year. 

I'm now wondering if he would be better off going to a private school where the staff/pupil ratios are better and he seems to concentrate in smaller groups but the person who assessed him felt that we needed to be very careful with where we sent him because they might not necessarily have the right expertise. 

Has anyone else had experience of looking into this?

Is there any advice anyone can give about what to look for when looking at any schools given the special needs our son has?