Are the school working with you/understanding, would a deeper conversation/meeting with them help before you make the move?
Is he on any medication at present that could be adjusted?
ADHD is often found in autistic individuals which would affect concentration. Autistic and ADHD behaviour can be confusing to understand as one minute the person is quiet and focused the next they are restless and seeking stimulation (of any kind, good or bad). Learning about ADHD might be useful.
School expect deeper concentration as children age so this issue may not have been obvious before now. School might offer to help such as letting him have 'brain breaks', walk around classroom, regulate himself however works best for him. Does he stim at all? Would he be comfortable to stim in school? There's a huge range of fidget devices available also that he could take in, some can be worn discretely such as necklaces etc.
Autistics find socialising difficult and I've heard children can't sometimes use violence, even on people they like, as they struggle to know how to connect. Working on his social skills can help. Are there any groups for autistic children outside of school he could join? It's often said one of the best things for neurodivergent people is to spend lots of time with other neurodivergent people.
Finally for you as a parent you might look to find other parents/support groups for those with autistic children if you're going through this alone. Sometimes your own family can't understand or know how to help so you have to look elsewhere. Well done for reaching out to this community and I hope you can find something that helps.