Does he help clean up after wees/poos? Not in a telling off way - but mine was hard - some children really are hard to do. And we started at age 3, very very hard until age 4, intermittent accidents after that. None for the past 3 months (touch wood) but he's over 5 now.
I will share my experience - in terms of him as a child he is easily distracted, talks a lot, enjoys what he enjoys and ignores what he doesn't. I did start to explain what manners were, what being rude was, and ask him what ears were for a lot 'listening' - right well mummy's talking time to listen.
I never got upset or frustrated but needed to break the boundary in terms of talking and listening. So with this and other topics if I was speaking and he interrupted and refused to listen I would stop him and say it is rude as I am talking, I need his listening ears on etc.
When he did a poo he helped tip it in the toilet while I explained that poo belongs in the toilet, he washed his own bum down, it took time and effort from him but I explained it's important it's his job and that if poo goes straight in the toilet then the job is much easier as he only needs to wipe his bum. Wees the same. Loading the washing machine, hanging on the line included. Simply an understanding of the work involved when accidents happen in clothes.
When caught short over the age of 4 I didn't do that, as it was generally that he tried but didn't make it - but at 3.5 with zero poos in the toilet and only intermittent wees and masses of washing I went down this approach.
There could be fear of the unknown in terms of pooing in a toilet which is why trying to open the communication is important. So you know if he really isn't aware if a poo is coming - or if he's scared of this process and change. By the changing of topic it seems a bit like he's reluctant/worried about it.
And speaking to a health visitor and continuing to speak to others helps, even if it's just because it is SO hard having all this washing and people saying how fast they did it etc which just makes you feel terrible. I really think some are harder than others but that it is worth perservering