Yeah it sounds from your examples like he definitely has something affecting his executive function and working memory.
This must be really difficult.
The company is obliged to make reasonable adjustments to enable this, but that does not include other people who are managing them making reminders for them
Well, it does if there isn't a system in place to facilitate a task list and reminders (which a bigger company would probably have). It is reasonable that all tasks directed to the person are entered into some sort of calendar or to-do list, and the calendar or to-do list reminds him automatically. But for that, OP needs to enter the task, and OP said there was no point bothering to do that? I'm not sure which sector OP works but adding every single task, no matter how small, into the system and assigning those tasks doesn't seem like a foreign concept to me (in engineering). If the manager seems very focused on OPs "negative attitude" and getting quite personal about it, it could be that he is seeing OP as one of the "change is bad" types? Or it could absolutely be bullying, I don't know.
But either way it sounds like the problem is actually between you and your manager's expectations? If your manager is looking for you to manage this person more closely, and you aren't doing it... is there a reason? And how are you communicating this reason to your manager (i.e is it in an effective way based on needs / constraints / business reasons or is it all personal to do with the employee?).
If you want to fix this and try to repair the relationship you could try setting up a meeting with manager about how you could implement a Plan, Do, Check, Act system that you all could follow.
From yesterday's example... you perhaps wouldn't have asked him last minute before the meeting because it would have been a planned task (could he have a standard list of tasks that need to be done the morning of a customer visit?)
Do... employee thought he did, but he didn't. Because all systems must account for human error, where was the check? Act, the flyer wasn't there so where was the action?
But regardless of the things that could have been done to mitigate failure in that exact scenario which may or may not have been cost effective... go to your manager, explain the mistake, and come prepared with corrective action that will prevent that failure ever happening again. New process: customers are handed a flyer and any documentation they need for visit on arrival by reception staff. (Or whoever is greeting them).
Again it could absolutely be that the manager is either incapable himself, scared of putting a foot wrong wrt the Equality Act, or bullying you. But it could equally be that he is looking for you to step up and take a more proactive approach, such as coming armed with solutions along side complaints?
But it's totally your call OP. I have no idea your level or what you get paid, so that's just one suggestion for attempting to fix this but equally I wouldn't blame you for wanting to find a new job if you have lost the motivation to improve things.