Annie, my ds was equally bewildered. He started last term, a few months before his third birthday, and after a few sessions going happily, became clingy, with a quivering lip, and at home would say, "But I've been to pre-school. I don't need to go again." I started staying there, uninvited. I felt really unwelcome, and the first time, had to wait outside! (I wasn't welcome in the main room, and the back room was being used.) I was consistently told that he was settling well, would be fine once I'd gone, and that he was just playing up to me. Grrr.
By the end of last term, ds was going reluctantly, I felt like a lemon down there, and like they just didn't 'get' me; they and I are singing from a different hymn sheet regarding separation.
So this term, we're having a break. The manager was taken aback at our cancelling, and - I thought rather patronisingly - said she hoped that dp and I had given our decision a great deal of thought, and were we aware of the benefits of pre-school for our son? Double grrr.
Ds has been much happier this term - at home/out and about with me and having playdates with friends - and only today started to talk tentatively about changing his mind about pre-school.
I'm totally with Flocci on this. If you're going down the school route, the most important thing is that your child learns to enjoy the experience of school and structured learning - which they'll be with for 13+ years - before dealing with the separation anxiety, in their own time. I only wish our pre-school - which, for the most part, I really like - were more informed about separation anxiety, and sensitive about handling it.
Annie, I can't believe they're thinking of getting a SN bod in for your ds. It sounds to me like he has a really secure attachment to his mum, and will separate comfortably on his terms, in his own time. This is not a SN issue!
Stay as long and as much as you can, weaning yourself away as your ds's confidence grows - or else knock it on the head for a while, and try again later on. I like Flocci's approach. Good luck, Annie.