I almost fell off my chair when I read about the OP's experience!
First of all, snus is not illegal to buy, import, own or use. There are snus products that don't even contain tobacco, just flavoured legal herbs.
Second, snus is considered a less harmful nicotine product (than e.g. cigarettes, vaping etc) and therefore advocated by some as an effective tobacco harm reduction product. Many use it temporarily to stop smoking, like nicotine patches. Earlier this year, UK health minister Joe Churchill promised a post-Brexit, evidence-based gov't review of the ban on the sale of snus by UK-based retailers, which science many believe are flawed. In fact, snus has not only led to Sweden, where snus comes from, boasting the lowest smoking rates in Europe, but more importantly, also reporting the lowest rates of lung cancer across the continent. Heck, just like with cigarettes, most Scandinavian kids have tried snus at least once between the age of 10 and 16, not seldom during a school break. Having said that, snus is still a harmful nicotine product in its own right, containing several poisonous substances that you don't want your young kids to be exposed to for health reasons, let alone addicted.
Third, it's not illegal to carry a product that you believe to be illegal. If you carry a pouch of oregano believing it's illegal, you're not guilty of "intent to possess an illegal substance", that's absolutely ridiculous and a perversion of the law.
Fourth, your son's mental health condition, ADHD, should've been a mitigating factor on this occasion, especially if he wasn’t on the appropriate medication at the time (why wasn't he?). I'm absolutely aghast and disgusted by the school's decision to permanently exclude him, effectively handing him a life-changing sentence, without at least so much as a warning first. He's a 10-year old kid with ADHD for heaven's sake, who can't be considered mature enough to always understand the consequences of his actions, nevermind the world come crashing down on him for having made one little mistake as an impulsive little kid, probably under peer pressure. Seriously people, what are we, Nazis…?! Would it have been the same outcome if he would've been caught with a cigarette in his pocket, should be no difference whatsoever (fully legal, non-controlled product, just age-restricted, not like it's weed or anything).
Fifth, I think you should take this all the way without giving up, preferably with a lawyer by your side, as you shouldn't stand for this old-fashioned, zero-tolerance BS. Having ADHD, your son obviously needs the appropriate care, supervision, attention and medication to make it past the most difficult, impressionable teenage years of his life, when the risk of ending up in the wrong company (where snus would be the least of his problems) is the greatest. However if he copes well on medication, I think you should argue that it's completely wrong and unethical for the school, regardless of their rules, to hand out such a harsh and definitive punishment to such a young child , who's very actions on this occasion could be a (text book) symptom and explained by his medical mental-health condition, and that he, like everyone else, deserves a second chance, under which he also commits to remain at all times under such medication that allows him to cope with all other academic and social requirements of the school. Percentage-wise, there's at least one pupil with ADHD in every class in the country and most of them cope just fine if identified and dealt with – however it's only fair and human to cut these guys a bit of slack during the "adjustment" period. I wonder if media would run with a scoop of a school that comes down with such punity on kids with hidden medical conditions like this (e.g. ADHD, ADD, DAMP, autism, Ausberger, bipolar etc).
Finally, the best of luck to your son on what sounds like a shitty start to the rest of his life.