Wow, thank you all so much for your replies.

It's reassuring to know we're not the only ones. He doesn't care about sitting in a dirty nappy (thanks for the suggestion about going slow with changing him though). We did try the 'bootcamp' style thing but found it made him more resistant and he would get very upset when we made him sit on the toilet so often. We've bought him his own jocks but maybe we'll try him in his big brother's jocks! When we put him in jocks he just wets through them and doesn't care. Argh!!

Thank you for all your suggestions.