sphil you have my sympathy...
I found wall.e toy worked brilliantly as an incentive to get ds to start peeing in the toilet/potty or to just pee, he was holding on for hours to start with, I was getting really worried about it, to start with ds would not pee, so I trickled some water on his legs and he couldn't hold on...
then I started standing him in ankle deep water in the bath (ds is 4.3 and not keen on baths) holding the potty in front of him, lots of praise for success! so I would do this 3 times a day (after breakfast, lunch, tea), this really worked, wasn't often enough for ds and it has just progressed from there.
very slowly, loads of encouragement, and I try not to ask him if he needs to go even if he is holding his willy, sometimes we just go to the loo together and he will go for a pee, I say nothing, he needed help with the process (I was trying the backward step/physical not verbal prompting thing I learned at pecs course)
when we have gone out, to start with it was very stressful, because ds didn't want to pee anywhere, again he would hold on until we got home, even if he was really needing a pee.
5 weeks on he is becoming more confident everyday, will sometimes say he needs to go , and has peed in loos on the train and two public loos, even though he hates hand driers... loos without hand driers are great.
now we are working on poo, which ds is reluctant to do, we have him on senna and lactulose, he gets a pull up for poos/nightime but will still ask to go for a pee, most times he needs.
wall.e was a great motivator/incentive/reinforcer... is there something your ds would be motivated for like that?