Schools in England special education needs!

Basically my son is 7 and in year 3 at primary school. when he started school his first teacher told me she felt he needed extra help and would continue to need extra help. But we moved areas over a year ago and I explained all I needed to the new school. Now last year his teacher was great and had given him extra help and at the end of the year she had said he had made great progress! When he started in September his new teacher had said he was doing fine, he is a little behind but motivated! Now my issue is I have now received an email from the school “for the attention of all parents of children with special education needs and disabilities” I thought I had got it by mistake so called the school to be told my son is on the sen register but they didn’t know why and would talk to his teacher, they also asked if I had been told by his teacher? I explained no! I feel really upset now as I knew nothing about it and still don’t really! Does anyone know anything like what the school should have done or if they should have told me?