OMG how ridiculous - I am laughing at myself as I wrote a really long and probably helpful response but then went off to look at another tab, left it for (probably over an hour) and then realised I had two MN windows open and closed one, then wondered how this thread was going and realised I never even posted it! There's classic ADHD for you.
Anyway the gist was - you can't manage his ADHD for him. He has to want to manage it himself. That also doesn't mean that you just have to put up with stuff which is making your life difficult. You have the right to ask him to make changes.
Russell Barkley is the leading expert on ADHD and his resources are well worth looking at. In particular his four pillar approach to treating ADHD. No one of these is enough by itself, you need as many of them as possible.
Diagnosis (in particular, to check for other conditions which might be causing the same symptoms)
Education - the person with ADHD should learn as much as they can about the disorder. So should people living closely with them. This helps with the situations where you're upset thinking he doesn't care when actually he cares very much but the way he shows that will be different to a neurotypical person. It also helps him understand why certain things will help and others won't (for example, willpower/ "just trying harder" is basically a myth. I actually think this is true for everyone but it is especially visible in ADHD, because most people are able to equate willpower with strategies and planning which people with ADHD will struggle to do, and it's the strategies and planning which really make change).
Some good resources are Russell Barkley's talks on youtube, his book Taking Charge of Adult ADHD, the book "So I'm not crazy, stupid or lazy?!", the Youtube channel How To ADHD, some podcasts (ADHD Essentials, ADHD Rewired, ADDitude Experts) and Russell Barkley's ADHD Report which is free to view online and summarises all the most up to date research. (Sadly ended December 2022 but you can still read the old issues).
Medication - stimulant medication helps 70% of people with ADHD and for the remaining 30%, nonstimulant medication helps 90% of them. So 97% of people can experience some improvement in symptoms with medication. It is not accurate to try one medication once and then say "It doesn't work for me" - it is a process with trial and error and there are different stimulants, there are also non stimulants, there are different brands of the same stimulants, there are different formulations (extended / instant release for example). Medication does not completely eliminate symptoms but it helps with everything else. There are also "indirect" physical interventions such as exercise, getting enough sleep, treating other health conditions, cutting out other drugs (nicotine, alcohol, cannabis etc) all of which are much easier to keep up with when medicated. Of course it's a choice if you don't like/want medication but it seems silly to me to dismiss it if you have not tried all of the options.
Accommodations - these are all the tools like have a routine, make lists, use a planner - that often do not work well for people with ADHD (medication can make it easier/more effective to use these tools) AND extra things that maybe people without ADHD would not need, for example, I tend to arrange tasks that I start, whether that is a jigsaw puzzle, craft project, decluttering project, sorting the washing to put away - I figure out how to deal with interruption first and set it up so that WHEN I get interrupted/distracted, the task can be left in a state that I can understand it when I come back to it. A neurotypical person either would not get distracted from the task, or when they do get distracted from the task, they would hold it in working memory and understand/want to come back to it as soon as possible. What happens for me is that once my attention is off the task it no longer exists/is no longer interesting, and I might come back to it, but the possibility is remote. So to use the jigsaw example: If I was just doing the jigsaw on the table, then I get called away, I would not come back to it. The table now looks messy, and I'm not registering "jigsaw", I just register "stuff is here" so I would pile more stuff on top of it. The table is now out of use for anybody else and I can't even easily get to the jigsaw now either. When I do eventually clear the piles of stuff from the table, all the piles of sorted pieces are mixed up uselessly, there are bits which have fallen on the floor and been eaten by the hoover or kicked under the sofa or people are getting annoyed by finding them everywhere, someone left a banana skin on top of another piece and it has moulded onto the table. Even when I come back to it my previous progress is all undone and it's made life worse in the meantime because it was annoying people and we can't use the table.
Simple solution in this case - a jigsaw board. If I need to leave the jigsaw I fold it away and can store it neatly behind the sofa so it isn't in the way, my progress doesn't get undone and I can easily resume it at any time whether that is 5 minutes or 5 months later. But I basically have to treat ALL tasks this way and come up with mental "jigsaw board" type solutions for everything, because otherwise my entire life is like that dining room table with stuff piled on top of stuff and all the things are getting ruined by the weight of other stuff and it all feeds into itself and it's a cycle.