It's become a power struggle. You're trying to control his body and he's not having it. Time to make potty training fun, rewarding, and his idea. Since he's basically trained but just hasn't agreed to do it for you as well, try Terry Crane's Potty Training in one day. We were in as similar situation with our daughter. Did the potty party and kept up the rewards until she lost interest in them (about 2-3 weeks) and just used the potty for its own reward. We've never had a problem since. It's a lot of work preparing for it and consumes basically a weekend (one day for the party, and another to solidify it) but it was worth the effort, especially since it was so short-term. Good luck
Updated
It's become a power struggle. You're trying to control his body and he's not having it. Time to make potty training fun, rewarding, and his idea. Since he's basically trained but just hasn't agreed to do it for you as well, try Terry Crane's Potty Training in one day. We were in as similar situation with our daughter. Did the potty party and kept up the rewards until she lost interest in them (about 2-3 weeks) and just used the potty for its own reward. We've never had a problem since. It's a lot of work preparing for it and consumes basically a weekend (one day for the party, and another to solidify it) but it was worth the effort, especially since it was so short-term. Good luck