D.P.
When he cries, go to him, but do not pick up. Soothe or settle him the best you can in his crib, or just sit in the room with him until he falls asleep, then leave. The more you pick him up, the worse the problem becomes. Honestly, you may have to listen to a little crying until he readjusts to his former sleeping habits. My son still does this type of thing from time to time, but we stay firm on how to handle this, and in a night or so he is back to sleeping through the night just fine.