Ahh, our ds1 does not appear to respond to bribery either, but in the end I decided to start a star chart anyway, including weeing and poo-ing, and he got lots of stars for wees (he was still pooing in his pants at the time). He didn't show any interest in it until at the end of a week I counted up the stars with him and said Oh look how well you've done, you've got enough stars to go out to the shops with us tomorrow and choose a new toy!' (i.e. I didn't decide in advance how many he needed, I decided I'd tell him he had enough, no matter how many he had). Then we took him out and gave him a choice of 2 or 3 cheap cars and he picked one out and was so chuffed. After that, more interest in the star chart. I then kept reminding him how many more stars he'd get if he pooed on the loo too. We also promised him a more expensive, special new toy from the potty fairy' once he was doing all his wees and poos on the toilet, which he is now apart from some wet accidents so we've just given it to him. We chose something we knew he'd really want and told him what he'd get and showed him a photo of it in a brochure, so he got quite motivated about earning that.
IKWYM about him having to know his actions are wrong - I struggled with this too, he just didn't seem to think it mattered. I did once put him on the naughty step and I did shout at him . I don't know if it made any difference one way or the other TBH.
I think peer pressure does play a role here, ds was dry at nursery almost immediately but not at home for ages. Do you ever have other children round or visit friend's houses? I did feel that ds was taking note of his friends' toilet visits.
BTW, I would stop putting him in the pull-ups, I know it's horrible and you will end up with a mountain of washing, but think it gives the wrong message.
Could it be something to do with privacy? At ds's nursery the workers keep their distance unless the child requests help. Do you stand over him? He may want to be private about it.
Can't think of anything else at the moment.