The problem isn't with the school. No child should suffer and no teacher gets paid enough to be pelted with chairs or whatever else. As a teacher I've been there - had my finger badly cut with scissors, almost had my hair cut by a child, been hit with desks, chairs, feet, fists and everything else. Had my windows smashed and class turned upside down. Been signed off with PTSD. It's no good.
The system is broken, none of the provisions are well resourced enough for the level of need that exists and as a result everyone is suffering.
Most behaviour units closed, meaning children with behavioural issues moved to the communication units. So the children with communication issues have been pushed to mainstream where they're not being well supported. And the staff everywhere are fried.
I think yes, if other children are being hurt in any way, exclusion is a good option from the schools point of view. However the fact there are so few places where children like your DCs needs can be met is not ok. And it will likely be a difficult road for you.
If I was you I'd contact your local MP or council rep, and cc the head of education for whatever council area you are in on the emails. Explain the situation, quote some local policies and laws about education provision, and ask what their plan is. It's the most effective way of getting things done. Your child is entitled to full time education and I don't think they are legally able to exclude him without an alternative placement (I could be wrong there, I know that rule applies where I am).
Its a very difficult grey area for staff when a child is violent towards other children, technically they can't lay hands on to stop them, but morally they need to do something. It's safer for everyone involved if your DC (especially as he gets older) is in a setting with less potential triggers.