Are you prepared to get really mean? Could he be
shamed into wearing the pull-ups?
How about every time he pees, you take a photograph of him doing it and the trail he leaves behind him. Show him the photos - lots of them. Tell him that you will show them to
everyone if he doesn't wear the inco pants, because you are not prepared to spend all your life cleaning up after him.
Would that 'get' to him? You know his personality and his 'triggers' and I don't, but there were times I had to get really mean with my mum because otherwise there would be total refusal.
Resorting to the threat of public shaming is awful... but it's not as if you're actually going to do it! (And let's be honest, you've tried all the reasonable ways and nothing has worked.)
Fingers crossed for you - no one wants to live somewhere that smells like the Gents in the park
so you have to find a solution to this...