The book Big Baffling Behaviours by Robyn Gobbel to understand their nervous system reactions and how to respond to different points of it. It's the only book I've ever read that actually works for the really dysregulated behaviour. Ignore the bit where she vaguely insinuates that if your child has a sensitive nervous system, you might have accidentally traumatised them without meaning to - she has a trauma background, but ND children often have sensitive nervous systems and/or are constantly getting overloaded because of sensory processing differences and the fact their abilities are so out of step with adult expectations, many of which are invisible to the adults. Aside from that one sentence, the book is incredibly helpful.
ADHD medication was a game changer, completely turned everything around.
Parent Friendship Coaching, a social skills intervention for children with ADHD. (it's absolutely no use learning skills outside the moment IME but there are very few in the moment tools which work). Click "Download PDF" to read: https://chadd.org/attention-article/how-you-can-be-a-friendship-coach-for-your-child-with-adhd/
Stuart Shanker's Self-Reg to understand how aspects from multiple areas can build up to overwhelm a child's systems and capacity to cope. When it seems like they go from 0-60 over very little, it's more likely to be a straw that breaks the camel's back situation - they're probably hovering at 55+ most of the time.
For any of the sensory interventions to work, they need to be offered well before he is in the explosive state where he is breaking things. This is what DS2's school said as well "But he can't have that, he will throw it at people" he was basically in fight or flight mode constantly at school. A sensory fidget or a movement break is going to do jack shit in that situation, it's like turning up to a house fire with a water pistol from the £1 shop. We did speak to an OT who let him try a wobble cushion and a weighted vest, and I think the vest plus ear defenders probably would have helped him as proactive supports in the classroom, because one day when he was supposed to be catching up on his writing during an RE lesson he wasn't paying attention to, they stuck him in ear defenders and told him to get on with it and he LOVED this because he said it made everything so much better. But they wouldn't make them available to him all the time, and even when I asked and they said he could use them, I couldn't convince him of this because I think the way they were available was not explicit enough for him to understand.
I actually think his verbal understanding was (and still is, at 7) way way lower than it seemed from the complexity of his speech. Part of this was that he really struggles (still) to focus for long enough to follow a narrative, and actually any way I could get him to engage with narrative (ie, stories) has been helpful. So TV programmes, often those aimed at a younger age group (at 5 he loved Numberblocks and Peppa Pig) and books, the kind of book he will enjoy is getting harder to find but I did have a talk with a school social worker (they do a sort of pastoral role where we live) and she really strongly encouraged me to keep trying to get him to engage with stories in whatever format possible, and I do think this helps a lot. He now prefers nonfiction like children's documentary type programmes or factual books, but he is drawn to things like gaming videos on youtube or videos which are just collections of jokes/memes/dubious facts and I try to reduce the exposure he has to this kind of thing because I think it's unhelpful. (The gaming videos I find OK but only certain creators).
At home, boundaries could not be drawn as and when by adults - they had to be really clear, consistent and highly predictable, ideally routine-based rather than any decisions being made in the moment if possible, and basically any behaviour which occurred outside of the predetermined framework is just deescalated/ignored. Declarative Language is roughly the way that I had already worked out by trial and error to speak, so that is a useful book and it is a very short one and can be shared with school (though I doubt it will do much unless you happen to have a teacher who is literally asking for resources). It is better when household rules apply to everyone equally or follow a clear predetermined structure, and you are not trying to pull rank or engage in power struggles to enforce compliance. It's kind of hard to explain if you haven't come across this parenting approach before.
The background of Ross Greene's "Kids Do Well if they Can" and the ideas which come with his method are excellent but there is no way that I could actually have a productive conversation with DS2 at 5 and unmedicated about anything in the way the method suggests. I was still able to take things from it but I would not put too much store in this currently. The things which are helpful are the idea of triaging which issues to try to problem solve and dropping as many others as practical in order to reduce pressure, and considering predictable issues proactively and working out what might be able to be put in place in order to smooth some of it. Being proactive is a huge help, and although it is exhausting, it's not really more exhausting than dealing with the fallout in the moment because reactive parenting just adds fuel to the fire.
Trying to observe your child to see what they seem to need to feel safe/what tends to trigger and upset them is also really helpful. Two things which are important to my DS are being able to know what will happen when, so things like social stories, now and next boards, visual timetables, are really helpful to him. And the other thing is that he's very sensitive to sound and sound is an incredibly important part of how he perceives an environment, so knowing what to expect in terms of sounds, e.g. by showing a video, or allowing him control over sound e.g. by letting him use ear defenders, can help him a lot. But I didn't know that when he was 5, it's something I've observed more recently.