D.C.
I just had to go through this issue also, my ds is 3yr 4months old. We had gone past a pre-school and he said he wanted to go to school. So we stopped and he visited, he really like it. I just told him that if he wants to go to school like his big brother, he had to use the big potty all the time because they don't let boys go that are in diapers(we never used pull-ups).Unbelievably that was it!! it worked. Just come up with things that entice him like going to the zoo, library or a special park maybe that will work? He also might be afraid of it disappearing(mine was) I told him it feeds the worms and they need to eat. Just keep in mind that this is almost the last thing he has control of in his little life and its hard to let it go. I wish you the best.