MommyD -- yes, re losing the bed: that's what i mean about making it explicit. your DS is old enough to know about consquences/actions. he's not sleeping/lying in quietly, so he's not a big boy, so he's going back into the cot. if he continues to climb out of cot, that too is babyish behaviour and will delay the return to the big boy's bed, which you say he loves. fantastic, he should be fairly motivated in that case.
how does DS react to star charts? i'm a huge fan (dd quite keen too). get him to choose the stickers himself in the shop, help you draw up the chart.
if this doesn't work, could you borrow a grobag for a while, so he's not so mobile in the cotbed when the sides are up? again, he would HATE it initially -- explain why he has to sleep in one, tell him it's up to him to prove that he doesn't need it, by showing you that he can behave like a big boy... it's a lesson in self-determination, really!
also i think it's useful to have a good talk at nighttime about how, if it's dark when he wakes up, that means it's still sleeping time etc etc. give him a sippy cup of water and tell him its there if he's thirsty, have a sip, turn over and go back to sleep...i picked up that tip from another thread, and it worked for us