J.C.
For me it was when I was ready! not him as much. One day (at about 2 yrs old) I said, i'm not doing diapers anymore (we did cloth). I went down that day and bought big boy underwear, which he loved. We spent at least one whole day training. And we never put him back in a diaper, except at night. So once he realized that, after a few accidents, he was trained within a few days. You just have to find what motivates him. And if he's scared of the big potty, get him a cute little one and maybe that will solve the problem. Good luck!