I can't offer a solution, but I would suggest trying "it's time for breakfast/coffee/lunch/tea/dinner/bed, so now we wash our hands." Don't mention the toilet - just find a reason to go to the bathroom. Once you are there, encourage him to use the toilet, as if it's only just occurred to you. Easier said than done, but you may find that you can gradually get him into the habit of being assisted, even if he grumbles about it. Or not. It may not be possible for a family member to do it. He's the senior male and it probably offends him to be reminded as if he was a child.
The rest of my post describes the progression we have experienced.
My mum, now aged 91, developed sudden onset vascular dementia 9 years ago. She was continent for about 2 years afterwards. She gradually lost the awareness that she needed to go. For a couple of years it was sufficient to take her to the toilet every hour or so and hover outside to be sure she had dressed again and washed her hands.
Then she became incontinent of urine and needed pads and then pull-ups. She still had some sensation of a full bladder but that has now completely gone. She can be toileted, washed and dried, but will often release the contents of a refilled bladder just as she's being put into fresh pull-ups. Her carers encourage plenty of fluids but it is still a mystery how much urine is produced in 24 hours.
It always seems to be far more than she drinks.
Her carers have a system for managing her bowel movements. She is put on the commode when she wakes up an hour or so after breakfast. If nothing happens they try again after lunch. She sits there until it happens but has no awareness of it. Her body seems to know what to do without her making a conscious effort.
I suppose it is like when you start to potty train a baby. You get to know their digestive rhythm and provide the relevant toileting opportunities.
I wouldn't want to have to do that with an uncooperative adult, so you have my utmost sympathy. My mum is compliant with her carers and therefore very seldom has a bowel movement in her pull-ups. Not nice for her or the carers when it does happen - a bigger clean-up job.
