How are you going about getting him to sleep to begin with. rocking, screaming it out, ect... ? My daughter went through this. We finally let her scream it out. I checked on her every five and then ten minuets. How old is your child? He may be at the stage that he is learning about object pertenence. (Can't spell sorry) This means that if they can't see you or anything that it is gone forever. You leave the room you are gone forever. Thats why if you do the screaming out method you let them know you are still there every five and then ten minuets. You have to build up to the ten. And it may take a few nights. Don't pick them up just rub on the back and say mommy is here its ok.

A good way to practice this during the day is playing hide and seek with toys. Or go out of site for a short period but keep talking and then reappear.