N.D.
I dont think there is anyway to make it less painful. This is a problem with co-sleeping. At what point does the baby go into his own bed? Your son has been used to having you snuggle him for 2 years, all his life, and now he has to snuggle himself. I would think you should start with naps, if he has to go to sleep with you rocking him at naptime, put him in his crib as soon as he goes to sleep. Then he will wake up to his own bed with his toys and snuggly animals. Dont rush to take him out when he wakes up. Dont let him scream, but tell him just a minute. If he starts to fuss, make believe you have an important job to do in his room and leave him in the crib. He needs to think of his crib as a safe comfortable place to relax in.
IMO crying=it=out is too harsh for a child this young. He needs comfort. I am also not a fan of co-sleeping for this reason, when a baby is born its a huge shock and they get used to sleeping alone right away. Then they are returned to mommy's outside womb every night, instead of their own little environment.