Aggressive behaviour at school and at home

My son is 5 years old, verbal and has a diagnosis of ASD. He's recently started year 1 in mainstream (has EHCP) and is really struggling and using aggressive behaviour to cope. For example, if he wants something is the classroom he will hit another child and take it. Last week he hit the headteacher in the face when she tried to calm him down. He lashes out at his sister at home as he struggles to share with her.

We are using 'time out' and taking away privileges and explaining other strategies to regulate his emotions like clasping hands, finding a quiet space or deep breathing when he feels angry or frustrated.

My question is have any parents found ways to prevent this behaviour. What worked  well?

I know all children respond differently but at this point I'm happy to explore alternative ways to find the best technique that resonates with him.

Thanks

Emma