I'm an ex-secondary teacher and now work in children's mental health. I alao have three children going into years 11,10, and 7 this week.
My middle one really struggled with the transition to secondary, but he is now going into Year 10 and loves school.
I would say this:
Everyone is nervous, even the teachers . I never went through a September in teaching without feeling utterly sick for the first few days.
Teachers are generally really lovely people and they go into teaching because they want to work with children and young people.
You will find your people - not straight away, but unlike primary school where you are kind of pushes into friendships because you're in a class together, at secondary you will get a chance to meet people who like the things you like, and who really get you.
Take it a lesson at a time - he doesn't need to think about getting through the whole day, just concentrate on the lesson he is in.
For you, as his mum, I would say be as positive as you can without undermining his feelings. Tell him that you know he's nervous and that is normal, but that the this is the hardest bit - after tomorrow it will start to feel easier and easier. Tell him that you are excited for all the new things he will do, and reiterate that this is a time full of hope and potential.
As parents we need to tread a balance between empathy and confidence. "Yes -it feels big and scary, but I know you can do this and you're ready for this next step, which is going to be so exciting and fun".
My daughter who is going into Y7 later this week had an awful time at primary school and never went in happily (she's autistic and really struggled with the environment,). She's told me this week that she "hates school", but I've told her that she doesn't hate "school," she hates her primary school and that is over now.
She is going to walk in with her brother and a few of his friends - has your son got a plan about who he is walking in with? If not can you drop him off tomorrow (lots of Y7s will be dropped off on the first day) and then get him to make plans to walk in with someone tomorrow?
If feels like such a huge step, and it is, but they will find their feet and if he can't think of any positives yet, just remind him that it's a short week this week!