Has the SENCO been involved with the transition from Primary School? I was the SENCO for a large comprehensive and had responsibility for working with all neurodivergent students regardless of their academic ability and making staff aware of any specific mannerisms or needs in the classroom.
I had a quiet room that any pupil could access at lunch times and many bright autistic students went there. Some had Time Out passes to use if classrooms got too noisy, students could use them to come to my Support Centre for a few moments and have a chat with an adult if they wanted to. The library was also a safe place to go to sit quietly or go on the computer. One of my TAs ran an after school club for Minecraft and coding which appealed to some such pupils.
These things did not eliminate bullying I’m afraid but did give students coping strategies and gave me the opportunity to find out who was doing the bullying so I could work with the Head of Year to deal with the bullies. Your grandson is coming to the end of the school year so it is an ideal opportunity to look at his teaching groups for next year. I would suggest putting him in the higher sets, possibly higher than his ability might indicate, so he is less likely to be in troublesome classes. If he had good friends from Primary School, it’s a chance for him to be put in some of their classes for support.
It is totally unacceptable and your daughter is right to go in to complain, the school should be working to prevent incidents possibly through circle time sessions when the bullies can learn how harmful their behaviour is.
If your grandson has a diagnosis of autism ( apologies if I’m not using the current terminology, I retired in 2016 and I know things have changed) then if he has regular CAHMS input the psychologists there can work with the school.
Many children who are different need good coping strategies, safe places and empathetic key workers and the school can provide these to make his life easier.