M.D.
Sounds like your son is not ready. Don't force it or he may end up not enjoying bedtime and that will be a larger issue. It may also be safety issue if he's very upset or climbing around his room. You will have a "grace period" of 3-4 months after the baby is born since an infant can sleep in a bassinet. Try to be patient and if he still isn't ready, you might just have to buy or borrow another crib. Trust me, you don't want a toddler going crazy in his room or wandering out of his bed when you have a newborn and you're not sleeping as it is. When your son is ready, he will be climbing out of his crib and will be more cognizant of sleeping in his "big boy" bed. My son is 3 now and I let him pick out his sheets and a pig pillow - that helped him get excited about it. He was 2.5 when he transitioned...and he still tries to get out! It's not easy! Good luck.