11 Year Old Son's Transition To Secondary School

Please could we have some advice re my son and secondary school.
My son is 11 years old and has a diagnosis of autism and separation anxiety disorder. He started secondary school last week but only managed the first day. We realise now that he did this because it was only year 7's in that day.
He is too anxious to attend since, missing the first week. 
It's a big school, with1300 pupils, my husband and myself had serious doubts about our son's ability to attend this school but he was adamant he wanted to go, as all his year from primary were going there.
We have tried gently encouraging and being more assertive but he just can't do it. I don't want too push too hard because last year he stopped attending primary school for a term due to anxiety, and it was an awful time, he was so low in mood and developed a tic due to the stress . The only thing that got him to attend school again was a very gradual return, where he felt he had some control. 
Anyway I had a very unhelpful phone call with the secondary school, where they said you just need to bring him in, kicking and screaming if necessary. I told them I wouldn't be doing this and that a psychologist has advised us not to do this. I explained what he needs is a gradual return, reduced timetable.
The situation is complicated because even though the primary school finished the paperwork for his EHCP in June, we have just been told it was only received by the local authority last week. This means that it could be months till we have any information re this.
We have a meeting with the School on Tuesday, what are our options? If we remove him from the school,  can I home school until we have an EHCP and can get him into our desired school? Or if he has 15 days off school are the Local Authority forced to home tutor him? Any advice would be so appreciated, we are lost.