lookslikerain - sorry for being slow getting back to this thread.
I broke it down into tiny steps. Since I knew he could do it, I started by rewarding every wee in the loo according to an agreed rate. On the same table, I had things like behaving well in the bathroom (he was prone to start fiddling with things instead of doing what he went in for), washing and drying his hands properly and other things I knew he could do, but often didn't. With over £10 to put towards lego accrued, at the end of the week, I made it a little more difficult, next week. The rewards for what we knew he could do were smaller and new challenges were introduced, such as rewards for keeping his pull up dry for a whole morning. After a couple of weeks and some shiny new lego, I introduced a hefty reward for doing a poo in the loo, which turned into rewards for doing all his poos that day in the loo, by which point, he was confident enough to choose some pants and wear them for an increasing part of the day. The final hurdle was using his loo using skills at school, after which he got to choose his own final reward for trying so hard.
We didn't push him with getting rid of his night time nappy until the next summer, when he was going to get a new bed. We agreed that his last few weeks with his old bed were a good time to practice going without and get into the habit of going to the loo if he woke up in the night.
Goodness know how we'll do it with DS2, though. We're considering it a huge enough achievement that he's beginning to change his own pull up.