Our daughter also slept w/ us since birth. At age 2 we moved her to her crib mattress on the floor next to our bed. It took a while but eventually she stayed there. (She woke around 5 every morning and climbed in bed w/ us.)
A couple things that helped us: I gave her "my special blankie" for her to hold onto, I call it Mommy's love. Silly, but it helped.
A "glow bear". I am not sure if "Glo-E" is still on the market, but he has been a Godsend. He is a glowing bear that has a 10-15 minute light show cycle. Our daughter turns it on whenever she is a little scared.
Over time she has learned to fall asleep in the room by herself.
We did transition her to her own bed in her own room. There we let her sleep w/ a dim light on and bedtime music.
It can be done. It just takes time. Do what works for you and don't allow people to bully you and say your child will be in your bed forever. That would only happen if you allowed him to. :)
We are expecting our own little boy in a few months and he will be co-sleeping w/ us. So for all the hard work involved, we are going to start the process all over again. :)
P.S. (Sorry-this is getting long.) Does your son wake up screaming cause his pacifier has fallen out of his mouth? To me, fixing this problem would be my first priority. Our daughter always used to wake up screaming. She'd also wake up in the middle of the night if her pacifier went m.i.a. but usually settled back down immeadiately.
Maybe he is in pain? Have you talked to your ped. about it?
PM me if you want to talk some more about transitioning. :)