You might have to just let him cry it out and he should get used to sleeping in the pack n play.
Either that or he's not comfortable sleeping in the pack n play playpen. If he doesn't want to sleep in the pack n play then I would just not force the issue and have him sleep in his crib instead.
Is there a reason he needs to sleep in the pack n play? If not then just have him sleep in the crib where he'll be more comfortable.