First make a big deal out of you being excited that he's going to be going to sleep in his big boy bed now, all the time. Then create, if you haven't already, a soothing bedtime ritual that will both condition him for bedtime and soothe and calm him. This ritual ( story reading/telling, brushing teeth, washing up, getting into pajamas, songs, etc.) should take between 20-30 minutes.
For the first few nights you put him in his bed alone to go to sleep, I would make the bedtimes somewhat later than usual (but make sure he does not fall asleep anywhere but his bed) to insure that he has the best chance of falling asleep quickly. Pat his back, tell him you'll be in the room next to his while he falls asleep and that you'll see him in the morning. You may have to sit next to his bed for a while until he falls asleep, then leave. You should gradually taper this routine off. For detailed advice on how to deal with sleep disturbances that may surface consult any of Dr. Richard Ferber's books in your library or bookstore. Good luck and sweet dreams to all.