J.C.
I have just gone through this exactly with my son. He even got constipated because he didn't want to go on the potty. He was urinating without accidents within 2-3 days but his BM took him about 3 weeks. He also asked to be put in a diaper. When he then went we would take it to his potty and put it in there to show him and then flush it in the big toilet. We used a lot of verbal encouragement and made him proud for going.
If your son has the urinating part he will probably catch on pretty soon.