My ds is 3 1/2, and was tt just before his 3rd birthday. We waited until we ran down his regular nappies, bought 1 pack of pull-ups for night-time (coz they looked different from his regular nappies) and told him there were no more nappies, we'd run out and seeing as he was a big boy he could wear jocks. This seemed to work as no amount of searching under his bed or in the cupboard could produce any!

He did still have them for poo though for abour a month, (I had a stash in our bedroom that I'd bring out just for this) I don't know what it is about boys but friends have had the same problem with boys and poo on the toilet. My ds would hold it for up to 2-3 days until his tummy was hard as a rock and he was in terrible pain. We ended up putting one on him for a poo only during the day, then straight off, watching him like a hawk to make sure it was poo he wanted it for not wee! It wasn't until he was at my parents one day and he told his nanna he needed to do a poo that it actually happened on the toilet! After that there was no more issues! I think the first time is the scariest for some reason. He was fine after that!

To reward for doing a poo on the toilet, we bought a much wanted toy and put it on the fridge in clear view but out of reach. He knew that's what he'd get when he did poos on the toilet.

Good luck!