I have to say I do feel sorry for the teacher (did you say she was an NQT?) and a little sorry for your DC's peers as it is never nice to see your friend kick off and throw chairs and hurt their teacher.
It is really important that you have a proper conversation with the teacher and the new HT. It would be interesting to see if this is the new HT asserting her new authority and having a 'zero tolerance' approach or if this is the last straw in a series of unreported events.
Really good advice from Tiredness and EBD. I haven't much to add other than it must be really upsetting for you and you have my best wishes. It's really difficult as a Mum to have to take on an institution in defence of your little son.
If you don't mind me saying, there is a sense of 'he only did this because...' from your posts. Sometimes parents can take on a 'spokesperson' role for their kid's behaviours and reactions at school which isn't always helpful. Sometimes the school need you to hear 'this is really unusual behaviour and we are worried' as much as they need you to help them work out why your DC reacted as he did. Teachers usually look at at a child's behaviour in the context of what everyone else's behaviour is like, ie. child acting out vs. behaviour of everyone else in the class. Sometimes the hardest thing to effectively communicate to a parent is the unusualness of a particular behaviour relative to what they usually see from children of a similar age.
I'm not sure you appreciate how unusual it is for a 5 year old boy to react to a simple 'tidy up' request in this way. I say this as an infant teacher in inner London schools for many years, I've only ever experienced a handful of kids who acted out to this level, and usually it was a) a precursor to a diagnosis of ASD or some other special need or b) symptomatic of a chaotic home life.
Sometimes the home environment is so effectively set up to understand and meet that child's needs that it's really hard to explain to a parent that the same approach can't always be taken in school as they have to effectively cater for 30 children at once.
Feel free to ignore if this is not helpful, but given the previous involvement of the EP, I wonder has anyone ever mentioned the possibility of a particular special need (e.g. ASD) to you? I only ask as you seem to have researched and ruled out the possibility. Has this been flagged in the past?
I hope your meetings go well this week.