The diagnosis does help, even if you don't want to look into medication - although I would say that the media has done a scupper with this over the last ~30 years most of which time most people (including me) were just absorbing it unquestioningly, so assume 90% of what you "know" about ADHD medication to be false - diagnosis helps because if you're only half of the mindset of "can they help this? Is it a choice?" then you react differently even if it's only half the time, compared to when you know, actually they genuinely have reduced capacity for that specific thing, or they do NEED that extra exercise or proximity to a safe/trusted adult, or space from their sibling, or whatever. I don't know how much is my own diagnosis vs his, but my relationship with DS2 has suffered so much less from his ADHD behaviour even though it is IMO worse than DS1's.
It's a bit like knowing that someone is deaf and you need to look at them when you speak because they lipread, rather than having constant misunderstandings because you think they are ignoring you or they get irritated with you because you have answered their question and they didn't hear you and the irritation, whichever side it is on, colours your next interaction which overall degrades the relationship between you.
Understanding him is one way to smooth out some of the interactions, which will make him happier. And there are ways to discipline but they are different to what most people will be able to get away with. For example, most parenting advice places the utmost importance on reacting to behaviour in the moment. For ADHD, sometimes that is true too, but a lot of the time you are reacting to and trying to deal with behaviour 1 and before you have managed to do anything at all child is engaging in behaviours 2-6 which you also do not like and it's impossible to keep up. Or even worse if behaviour 1 happens to be something you're trying to praise and then you have to abruptly perform a U-Turn. (There is a good tip for this that I read in an ADHD parenting book (Step by step help for children with ADHD) which says say something like "Good sitting!" - as short as possible and including what you are praising for within the praise, because frequently they have already moved on to something naughty.)
But the other issue with disciplining ADHD behaviour in the moment is that commonly ADHD disruptive or provocative behaviour is a sign of dysregulation, and trying to combat this will simply escalate the child further. They are not even in a state where they can actually take the information in at that point. I've been ill and a bit short on patience the last couple of days and if I have slightly raised my voice or given an irritable reprimand, DS2 literally covers his ears and starts screaming (we actually think he might be autistic as well as ADHD). Anyway if you are getting a lot of this kind of thing, then it can help to completely flip behaviour management, de-escalate in the moment (Mona Delahooke or NVR are good resources for this) and use a more proactive and specific approach. With this, you sit down away from the child when you have a quiet moment (lol sorry) and note down a small number of predictable behaviours or scenarios which can be tricky. Commonly these will be worded in terms of the negative behaviour e.g. refusing to wear seatbelt in car / hitting brother / complaining about turning off the TV (etc) so the first step is to flip each behaviour and consider what you actually want them to do instead.
Having considered this is instantly helpful because we tend to immediately notice what they do wrong but it is hard to notice when behaviour is neutral or helpful, often we just feel a sense of relief and don't want to interrupt but it is important to notice this, whether you notice it straight away or later on. Knowing what you want also makes it easier to communicate clearly.
Secondly knowing what you are looking for helps you set up a proactive plan for success so you can look at things which might be getting in the way, e.g. if you are having issues with morning routines a visual schedule and a space free of clutter and distractions to get ready can help. If the transition off the screen feels too sudden it might help to communicate more e.g. remind them at the end of this episode it's time to turn off etc. If they are often turning to physical violence they may need more support with communication and emotional regulation, and you could plan to work on these generally. (Ross Greene's books are really helpful for this idea).
Thirdly you can take the principle of shaping. This actually comes from dog training but it is really simple and clever - the principle of this is that rather than punishing unwanted behaviour, with the punishment increasing the more the unwanted behaviour continues, you look at what the child's behaviour is now, and what the behaviour expectation you want to get to, and you think OK what would be one small step from where they are now to where I want them to be. Then you change your behaviour expectation to that, for now. No punishment, mainly ignoring the behaviour (engaging with the behaviour is essentially giving attention to it, which children can experience as rewarding) but some kind of reward (praise or attention is fine or you can use tokens if you want) for getting to that one step closer. If the wanted behaviour is still not occurring, rather than add a punishment, reduce the size of the step. Once the step is occurring frequently, move to the next step towards where your aim is. This is really helpful because it starts where the child is and can build practice and therefore skills in a manageable way even without necessarily knowing why they are behaving as they are. There is a good video course on youtube for this, called Everyday Parenting Course by YaleCourses.
Russell Barkley is good for understanding ADHD. Mona Delahooke or Robyn Gobbel are good for understanding dysregulation. I believe a lot of what people refer to as PDA online is usually referring to any kind of dysregulation, not necessarily the specific kind of dysregulation which is a nervous system being triggered by demands. Stuart Shanker's Self-Reg has a helpful model which is about how everyone has five domains of stress and we are all more or less sensitive to different parts of these domains, but strain on any one area reduces capacity in the other four and dysregulation occurs when all the parts are overwhelmed at once.