We had to move my Dd#2 in with Dd#1 about 2 months ago. She was only 26 months. We popped her in a toddler bed. Like your boy, my DD#2 was a fantastic sleeper, we would pop her in her cot at 7pm, and not hear a peep out of her till 7am the next morning.
It turned yucky when we made the move to a bed. It was party time according to little miss. She was still up at 9pm some tnights. We too would lie with her, eventually she would fall asleep, but lying there for 2hrs each night was not something i was prepared to do....she was crabby in the day too. The lack of sleep was catching up on her, and eventually she got sick, gastro bug, run down etc etc. ..............so we bit the bullet and put her back in her cot (still sharing with her sister).

I just felt she was too young to understand the whole big bed thing (i.e we stay in it !!!!). Shes a totally different girl now. I know we still have the hurdle of a big bed later down the track, but for her, she just wasnt ready (i think we were, but she wasnt).......good luck.