When our DD transitioned she fell apart spectacularly, I would say think about a plan B and keep it in your back pocket
With her brother 2 years later I started prep a year before transition
Getting him to walk some of the way with a friend to increase independence
Made sure he was anonymous in that he blended in by making sure he had an up to date hair style, clothing, bag, shoes for a boy at secondary school
Provided a phone (monitored) similar to the other boys
We talked about life in secondary school, how to be friendly, joining in
If there are trends I make sure he has the item, like fidget spinners at the moment he has some mini ones which everyone wants to play with
Coming to the end of year 8 most boys game online to socialise, we changed console so that he was the same as his new classmates and could join in
Organisationally he's very dispraxic, I have a copy of his timetable so I can make sure he's organised for that particular day
We made sure he had duplicates of things like calculator, pen etc so he kept one set in his bag and one set on his desk at home so that he didn't forget them as this caused him intense anxiety
Even now I find he will pretend to be sick if he is anxious about a situation like forgetting homework
We talk a lot on the journey to school, often I explain that if he talks to the teacher about forgetting to do something they are much more tolerant and will often allow him time to complete the work
Every school is different but hopefully your son will find a safe place he can head to, for our DS it's the library and computer labs at lunchtime
Keep talking, I find helping them to understand situations reduces the stess for them
Just finally to reassure you, our DD is now fantastically happy and settled just not in a standard school, for her the route was EHCP and finally Interhigh where she can manage her extreme anxiety and still receive an education