I am sorry he is experiencing this.
You need a plan of action which starts with school policy. It is usually published on their school website and you need to put everything in writing. That way it is documented and you can show school that they are failing and what they are failing in.
Ds1 goes to an outstanding school with very strict discipline. The reason we chose it (and moved house a few years ago to get in catchment) was the strict discipline.
There have been incidents including being pinned to a wall by his neck but the school came down on the perpetrator like a tonne of bricks. So my son feels safe now. ie year 7s tested the system and found themselves in a lot of trouble including detention and isolation. They now know not to mess around.
My son had things taken from his pencil case in class and the children who are stealing have been moved away from my son. The system works because repeated behaviour is dealt with as soon as we raise it. That is how a good school works.
The only reason these issues are dealt with is because we report them. Not the one offs but the repeated incidents. Your son needs to report the fights, and if he feels he can't, then you need to. At the end of the day that is someone's child being attacked.
Secondary is a learning curve. The children that kick off have only lost 2 minutes of their break time in primary, they need to learn that in secondary the punishments are harsher for unacceptable behaviour.