Well first, I'm going to condemn those who condem co-sleeping!! Our oldest daughter had a fibral seizure when she was 9 months old due to a fever spike. That scared the life out of us, and from that point on she slept with us probably until she was close to 3 or so years old - until the time she about outgrew them. It wasn't what we anticipated, but we had to do what we had to do to make sure she was going to be safe, and there was no way to tell if she had a seizure if she was in her own bed.
Next, there are a couple of things you can try doing. First, prepare him for the change. Let him know "Tomorrow night, you're going to sleep in your special bed!" He should be able to do this in his own room. BUT she can do one of two things....graduate him to his own bed in their room, then to his room, or go right to his room. I'd try his room first.
When you do - make sure there's a night light. We have a humidifier that has a night light that shines up through the water tank. Also try to have a fan in there to create "white noise". AND when she does...have her put him to bed, and she sits at the corner of the bed facing the door. Every time he gets up, she puts him back without saying a word, and this may take quite a long time (hours) until he gets the idea and goes to sleep. It might take 2-3 days to get him used to the idea too. (This is what my sister did with her daughter, and it worked great.)
Good luck!