Struggling with starting a new school

My 13 year old son has moved schools this year to high school (where we live we have middle school so he has gone to year 9 at High School). However he was supposed to start on Monday and so far hasn't made it any of the three days. First day he overcame a lot and made it out of the house, but once he got to school took one look at the crowds and panicked and made me come home. He then suffered a huge anxiety attack late on Monday night and was so worried about trying again on Tuesday I had to just say we need to take the day to relax and calm down. Today he tried again to get to school but ended up being stuck on the toilet for 1 and a half hours (I think due to anxiety) so was late and has anxiety over going to school late.

The school are being great, they say they understand and are offering for the teacher to meet him at the car and giving me all the information he is asking for to help him feel re-assured.

However as a parent seeing him go through this anxiety it is really hard. Any advise on how to help ease his worries and my worries?

He did suffer in similar ways before he left school in the summer, and his previous school were supposed to have been doing his referral for diagnosis, however now he has left it appears they didn't do any such thing and so his referral and diagnosis has been delayed by nearly 12 months due to their incompetence.

I am obviously really worried for him as the longer this goes on the more worried he is going to be about starting the school he is already concerned he has missed the first few crucial days and this increase his anxiety more.