Emotional regulation is a skill, but it's very difficult to learn with unmedicated ADHD because the ADHD means that the "brakes" of the emotional regulation system are underpowered so the emotion tends to reach a point where it's actually pushed the person over into fight or flight (dysregulation) before they can even recognise what has happened. And because the "filter" bit that helps you work out what to pay attention to is also underpowered, it's difficult to notice the signs of emotional dysregulation before an emotion gets so strong, because they are lost in a sea of other noise of random thoughts, important thoughts/ideas, environmental stimulus etc. It can also be difficult to make and sustain progress on any goal because the cognitive centres are essentially cluttered with too much information and many people with ADHD struggle to prioritise, and therefore put effort into things in an inconsistent way which makes it difficult to maintain progress on anything.
And if the unmanaged ADHD is very bad and has knock on effects on the environment (e.g. messy house, or damaged relationships or bad habits) or things like self-care (nutrition, sleep, stress levels in general, any use of things like caffeine/nicotine/alcohol) then there is even more to compound the above issues. Particularly if there is generational trauma involved.
ADHD medication helps with things like the cognitive capacity, increasing it a bit so you can actually hold stuff in mind a bit more and it gives more power to the "brakes" and "filter" but - silly analogy, but if you think about when you get a new car, it takes you a bit to learn the sensitivity of the brakes, accelerator and clutch - it's not just automatic instantly. It's kind of like that with the ADHD medication. It makes the brakes more effective but you still have to learn how to use them smoothly. And a brain is more complicated than a car. And if you're not used to noticing the internal body sensations/signs of stress/dysregulation building up then you might not know what to look for. They are also different for different people so it's difficult to tell someone what to look out for - and dysregulation is rarely spoken about explicitly with ADHD patients so it's sort of if you either figure it out for yourself, or if you happen to come across it (I came across it in the context of children, but it's helped me hugely).
And the environmental aspects don't disappear overnight. Just like if you've built up a load of debt and then you start taking medication that lets you have more control over impulse spending, you're not going to take on more debt but you still have to pay off the debt you've already built up. And in the meantime that might lead to a situation where you NEED to spend money urgently that you don't have, resulting in needing to take on more debt, maybe at a high rate of interest if your credit score is bad. And if there is ingrained trauma or shame around this, it can trigger an emotional spiral even with the better emotional "brake".
So in terms of relationship patterns, someone with unmanaged ADHD and emotional reactivity is likely to be emotionally volatile towards both a supportive spouse or a spouse who is emotionally manipulative themselves. And if an initially supportive spouse has come to a point where they have sore points about the relationship and the person with ADHD has sore points too, then even with reduced emotional volatility, those sore points don't just disappear. They have to acknowledge them and this is using a skill which is new to them and they might have built up defensive walls around because that is a survival strategy of unmanaged ADHD. That might be more difficult to overcome and it might help to have some kind of therapy. But at least in theory with the medication it ought to be more accessible for them to learn better ways to communicate and manage their emotions.
Most people with ADHD who aren't diagnosed until adulthood have multiple issues like this, and it's not realistic to expect everything to become perfect overnight. I think though that the things you've read about emotional volatility becoming worse etc are short term, usually related to finding the right medication to settle on. Presumably he's titrating with a qualified mental health professional, in which case try to give the process time. Remember people who don't have problems tend not to post on the internet about it. The stats say something like 75% of people get on with the first ADHD medication they try. If that one isn't effective without side effects, then about 75% of them will find the other stimulant medication effective. There are two main stimulants used in the UK. This leaves about 5-10% of people who don't respond to the main two stimulant medications and then they tend to explore non stimulants which can help or it might be a combination of things helps or it might be that their diagnosis is more complex or something environmental needs to change before medication will be effective. Very few people are completely non responsive to medication, and while it might take a while to find the right thing, OTOH a lot of people do find the right thing fairly quickly.
But even after finding the right medication, he's still going to be using skills which are rusty to him and which he'll need to learn from the ground up. Some people are more aware and open to this than others. Some find it too painful to address that directly and tend to shut off to it, which probably unfortunately means they will make very little progress.