Sympathies OP. If it's any help at all personal hygiene has been a lifelong struggle for me and my brother, both raised in a dysfunctional home with chaotic parents, him ND definitely, me possibly so can offer some insight maybe - of course for your son I'm sure he's had a better upbringing than we had, I literally and embarrassingly genuinely did not really know or understand the basics of how often you need to wash to not smell when I went to uni as this was never modelled for us or taught whereas I can see you've really tried everything with your son. But what I can offer is:
I really don't think it's laziness as such, I do think both the actual physical/sensory experience of washing yourself but also the executive function needed to process and complete a personal hygiene routine as a task are much harder for a lot of ND people. I know that's really hard for an NT person to understand when the person may be very intelligent and able in lots of other ways, able to complete apparently more difficult tasks with ease but not stick to something as simple as showering every morning. But there is just something about mundane everyday jobs like cleaning your house that are hard for the likes of us where they're just not for others, that is the point that ND brains are wired differently. So I don't think simply nagging or shouting are likely to work. On the sensory side obviously he needs to work out what works for him but for me I've found baths are much better than showers, washing in the evening is better than the morning, and the plainest, least scented products are better than the super glitzy bubbly/fruity/floral products that often get recommended, but you say you've got him all sorts of things already so I am sure you're on top of that. It also really helps having a big fluffy robe or towel available to put on straight from the shower/bath as the transition between hot and steamy in the bath to cold and drippy wet is horrible for me (shudder). I think if you're trying to set expectations for him (which is perfectly reasonable when he's living at home) try and go with very small changes, things that are easily achievable rather than what you necessarily would consider ideal or even minimum acceptable, one of the things I struggled with most as a young adult was thinking that in order to be 'clean' I needed to shower twice a day using lots of products and complicated multi-step routines. I know that is normal for lots of people but it just felt so totally unachievable for me that I gave up on the whole hygiene thing entirely for a while. Whereas when I eventually gave myself permission (avert your eyes MN hygiene boil wash brigade) to bathe or shower properly 3 times a week and brush my teeth once a day it suddenly felt so much more achievable and less overwhelming and actually I can now manage much more than that reliably.
I'd really try and avoid making him feel ashamed or fearmongering or succumbing to catastrophising yourself - I know especially with things like his teeth you must be so worried but he's clearly not stupid, it's not that he's not able to understand what you/the dentist tell him, if he's anything like me and my brother he probably feels an enormous amount of shame and guilt over his appearance and smell and that will be why he's getting angry/irritable with you as the messenger of what he already knows, but clearly it's not effective in making change happen, we would more tend to spiral and be less functional if something happens like a loving family member intervenes than actually take action.
Things that can work really well, gamification can really appeal to ADHD brains, there are various apps that are designed for this very purpose, I know again it sounds so stupid that I'd need an app to remember to wash and do my laundry but what can I say, it helps. Maybe you can start a conversation with him about this, if he's into gaming he's probably heard of a few?
I also think he'll likely take advice and ideas much better coming from online influencers, there's loads out there now about neurodiversity and cognitive issues etc - I don't really know how you get him into them as I'm sure he doesn't want recommendations from mum 😂 but again maybe open conversations, listening and sharing ideas is the way to go?
Also just making small positive changes the easier choice is food, what you say about his clothes reminded me a lot of my brother, one thing we've done together that helps him is making sure he has a really big supply of clean, easy, soft and comfortable clothes always to hand, so that even on his worst days it's the easiest choice to put something clean on, even if he does literally nothing else. I know again this will be repulsive to many on MN, and that a dirty body will still smell even in clean clothes but dirty clothes on a dirty body definitely smell worse than clean clothes dirty body. If he can add deodorant as he's getting dressed better yet. Like I say, small, achievable steps.
Anyway hope all that helped and I'm not about to be flamed by all of MN for being minging now 😂