Back completely off. I respectfully disagree with the idea of negative discipline during potty training as it just encourages the refusals.
He is flexing his muscles, and for some reason, it seems it happens a lot with potty training. I know it did with my last two and nearly had me head-bangin'-bananas. I had to just let them be in control of it.
I reminded my daughter that this was her body, her potty, and her choice. I made her responsible for cleaning up her mess, changing her clothes (or pull-ups during the "I SHALL NEVER POOP AGAIN" phase). When she did go, I acted like she had discovered penicillin. It was a huge deal. We did not give her treats, but we did treat her like a hero. Before you knew it we had a potty trained child in a lot less time than it would have taken if we had butted heads over it, and it was a lot more positive experience for her.