Wysiwig, same as OP my DS one day just decided to wear undies, and he's been dry and clean day and night since then - obviously a couple of accidents in between.
Frustratingly we had been trying for 4 months and getting no where, lots of tears, anger, frustration of him just not getting it - so we stopped talking about it, went into nappies full time again (had been doing, pull ups, training pants, undies and nappies in that 4months) and after 2 weeks he came and asked for undies and we haven't looked back!

He got an m&m for a wee on the toilet, and gummi bear for a poo and a chuggington train for staying dry and clean all day.