Up until after year 2 I was the same.
Hated school and refused to go in. I was a very shy child when not around my own family. I would cry for hours every school day and it wasn’t very productive for me or my classmates.
The school ended up sending me home with whatever work books they were giving that day. I would finish the entire book in one day at home and then be on to the next one. I actually really excelled being at home with study guided by myself and my mum and was very much ahead by the time I went to junior school. On occasional days I would be able to cope with half a day.
The same issues reared their head in secondary school so I ended up going in for timetabled lessons, being allowed to opt out of group work, and going home in between lessons. I passed with very good GCSE’s and A Levels this way.
It might be worth considering if he is really ready for school yet and exploring other options for his education. For some children large groups are just impossible to feel comfortable in. Another year could change that, or maybe a different environment.
There’s no shame in admitting some children need to start later, or that some children just don’t cope in a school environment.
My mum had guidance from a next door neighbour who had home schooled a child, and that would have been the best option for my whole education looking back on it, but it wasn’t possible for my mum in secondary school, she wasn’t educated to a level she felt comfortable with. Me guiding my learning with some input from school was the perfect compromise.