Phew! Ok, I have read the thread. I can't answer everyone individually, but to answer a few questions:
His behaviour - I have never excused his behaviour to the school, I have always supported the school regarding sanctions. Yes it is low level, although I totally get that it's still unnacceptable, and have never said to him that it was. Until now, I have never allowd ADHD to be an excuse, although I'm justified to believe that it has some bearing on poor decisions.
Stage one concerns are handed out for practically anything, including forgetting books etc.
He struggles to behave, sometimes, in a way that the school deems acceptable due to ADHD. Any parent of a child with ADHD will pretty much understand this. There is way too much info that I would need to relate on this subject to impart, hopefully, a full understanding. It would take a seriously long time to adequately describe his behaviour and the reasons for it.
There is the assumption here that his behaviour was not low level. It was recorded and fed back to me as low level. I was speaking to a member of staff about behaviour concerns, and I mentioned whether there was a chance of him being excluded. She said 'We're a long way off that'. He was permanently excluded a week later due to 'the incident'.
I'm not suggesting that low level is ok, it's not ok, and I understand where everyone's coming from.
'If you are worried about a managed move because you think he won't behave himself for 3 months then it's your son's behaviour that's the problem....' - I am worried, yes, because he has ADHD, and he struggles in ways that NT kids don't. Certain behaviours are just him, others, not so much.
He is not a 'constant trouble maker'.
Yes, he has a formal diagnosis, but no ECHP.
Homeschooling is not really an option.
Thankyou Da1sycha1n 
I didn't buy the pouch for him, another student brought it into school. It was a nicotine pouch and my DS was told it was called snus, it wasn't snus.
My anger is directed at both DS and the school.
I definitely do not condone the use of age restricted products, and am seriously disappointed in him. This does not usually result in permanent exclusion though. He has never smoked a cigarette.
I got it wrong in terms of saying 10+ couldn't be excluded, I hold my hands up.
'ADHD does not cause children to be naughty' - it often does actually, depending on your definition of the word 'naughty'. This is a super strict school. Do you have a child with ADHD? If you do, do you know other children with this condition? It is classed as a disibility, related to cognitive function. It doesn't however, affect intelligence.
DS was on course to achieve 7's and 8's in his GCSE's, he was taking AS level maths. All this has been replaced by watching netflix and making wooden reindeer.
No, PRU's are not all bad, but the one DS is at has some seriously challenging students. DS was asked by a student 'Do you sell [drugs]?' on day 2, whilst the student stood outside for a fag break.
Thank you admission, I appreciate your post.
Thanks so much KlopTeeth, I will DM you, thanks for the offer, and thanks everyone who has replied so far, it's really appreciated, and I've read carefully every one.