Ok, firstly I think youre doing the right thing - putting your foot down and being firm that its time to TT

I have another idea - because he has never actually gone to the toilet, and doesn't respond to being asked if he needs to go - what about having an alarm (even the microwave timer?) that goes off every hour or so, and when it goes off its "toilet time!" and everyone goes to the toilet, whether anything comes out or not. You sit on the toilet for a minute (or something) and that way it becomes a familiar, normal thing - even if nothing happens.

But I think you need to keep it light too - how about blue food colouring in the toilet - and he has to make it turn green?!
Can you take him shopping so he can choose his own undies?

All the best darl