Hi T. -
If you are going to buy (or already have) a new bed for him (instead of a crib that converts) and if you have the space in his room, you can try this: set up his new bed, use it for reading him stories before bed, etc..but continue putting him in his crib to sleep for the first few nights. If he still takes naps, ask him every afternoon if he wants to try sleeping in his new bed. Continue to ask him every afternoon (and soon every night, too), but don't push him. Eventually, he'll get so used to the bed that he'll begin playing on it by himself, and will hopefully surprise you one of these days by asking to sleep in it. Once he does decide to sleep in it, though, leave his crib up for a week or so until he is good and used to it, otherwise he may get really upset.
Another thought - maybe have him tuck his stuffed animals into the big boy bed...And if you haven't bought a new bed for him yet, take him with you to pick it out. Also take him to pick out new sheets, pillow, blanket, etc..that way he's more likely to sleep in it if he got to help pick it out.
good luck
dana
ps - before you let him loose in his big boy bed, make sure that his room is safe for him, for if and when he does get out of his bed