Just wanted to add my voice to the negative consequences idea.
I sympathise with you so much. My DD was hell to potty train. Like a PP's child, she was very stubborn and very bright. She knew exactly what was expected but just... wouldn't. She wouldn't wet or soil herself either, she'd just hold it in all day (she must have been in so much pain) but couldn't/wouldn't unclench. It was horrendous.
Eventually, two months before her fourth birthday, I said enough was enough and we were going to crack it this time. We'd tried everything. Day one went predictably. And then, at bedtime, I got angry and said we were going to sit in the bathroom until she did a wee. And she did. And that was basically the turning point.
She still tried to kick against the whole toileting thing in other ways. She wouldn't use public toilets. I remember one very low point when we had to take her and her potty out of Pizza Express on her 4th birthday and into the car park so she'd do a wee
Honestly I cringe thinking about how difficult she made things, and the humiliating things we had to do to appease her.
But, she grew out of it. At five she's fine. Normal. No longer kicks off about using any public toilet of any description. Haven't forgotten how bad it was though!
So, if the matter of fact, no emotion, "don't care if you poo on the floor or not" approach isn't working, then maybe getting cross will. You don't have to scare the life out of him, but as user says, sometimes children need to learn that their choices are not OK.