but this boy has already been moved out of 2 other schools?
depends if he was permanently excluded, or if parents moved him JUST before the paperwork made it official. (Schools sometimes encourage this, for the follwoing reason) It used to be (and I think still is) the case that if a pupil is permanently excluded from three schools, then they are not entitled to main stream education, but have to go to a Pupil Referral Unit. These places are v v expensive to run, and some LEAs only have one, with a very limited number of places available. They are 'schools' but come with panic buttons & barbed wire (not all, but some), so pupils only go there as the absolute last resort.
IF he has been perm. excluded from two schools already, that is his next place to go. It could be the other side of the county, with no transport, other than LEA funded taxi (but some taxi firm refuse to take pupils there). You can see why the head has to have a cast iron record of events to ensure that this happens.
I am guessing that the boy may not be the best behaved in class? It may be that the staff don't enjoy his presence in the school, but at this stage, they have to do everything by the boook.
However, I have known cases where just one incident of bodily harm was sufficient for a pupil to to excluded. It sounds like this boy is 'borderline' for that kind of treatment, and that there are politics further up the education system which are affecting this.
Tbh honest, none of that is relevant. If you feel your son is under threat, go to the police. As a teacher, I sometimes encouraged this, as the school is soemtimes wanting extra help to deal with a difficult pupil, particularly if parents weren't supportive.
Do what keeps your son safe, let the school worry about the poitics behind this. I'm just trying to explain some of it, as it might help you to understand comments made by the head.
You can tell I used to do the real 'inner city gangs' type of teaching, can't you? Would just like to add, it's been a couple of years since I was actively involved in this side of teaching, so could be out of date, but I think the underlying ideas still remain.