we started sitting on the potty around 18 months ish. I'd put him on his potty when i knew he would have to go and then we'd sit there and read a book for a while. If he pottied, GREAT! If he didn't oh well, we just went on with our day.
At 2 he refused to let me put a diaper on him, while we were out going to Costco. I was SOOO nervous, but he did great and we had a few accidents, but not many and that was that.
I'd just keep on doing what you're doing. Sounds like you are following his cues and letting him lead this adventure.
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