my dd is the same way, she is 2 years 6 months so the same age as your ds and she just screams "pamper" until I finally put it on her. When she was 2 years 3 months she was fully day trained and in big girl undies, not even in pull ups anymore, then she regressed and we can't figure out why and though I feel like you that she should be trained, I can't force her to do something she won't do as she is very stubborn and the more I push the more she pushes back.
I agree with everyone else, maybe try waiting until after your next is born and show your ds that is is the big brother and only babies wear nappies, maybe that might help? I really wanted my dd day trained when our next one comes in March since she will be almost 3 at that point but I doubt it's gonna happen.
good luck!!!