Hi, I don't have any answers I'm afraid, this is something my husband and I are going through too - and it is enough to try the patience of a saint. I just wanted to say that I know, and I understand.
Until a few weeks ago, going to the loo was a fairly easily managed event. Now there is total confusion getting hubby to understand how to sit on the loo - and what to do once sitting there.
I have started using the kind of bottle
@Izzy suggested and that worked well. However he is now totally confused about that too and often just stands or sits in total confusion as to what is expected of him. He knows well in advance when he needs the toilet (he tells me each time) but we can have several attempts over several hours with nothing but looking around in confusion. It seems that he needs to be on the very point of bursting before any kind of clarity about what to do gets triggered, which leaves me seconds to get to him in time.
On the rare occasions I do get him to sit down to wee It can go all over the place because he stops pointing down half way through. I have tried the bottle when he is sitting, and sometimes a plastic 'p-shield' to help contain everything - these worked initially but now they often just confuse him too and he doesn't finish the job. He was doing ok standing up to use the bottle but now I find he is just standing, doing nothing for ages - and then losing his balance as his mobility is deteriorating too.
I've also got a developing problem now that, because he is more likely to stand for the bottle, he is not sitting on the loo to have a regular BM. I struggle to sit him on the loo and he stands straight back up again!
As I say, I have no real answers yet, I'm just persevering through gritted teeth and muffled cusses when it happens. I just wanted you to know that I understand and that you should not be so hard on yourself x