SCREAMING BABY!!!

My 16 month old just recently started to scream every time I lay him down for bed. Like he's fine if I'm in the room, but once I walk out he screams. And I mean LOUD. I just don't know what to do. The pediatrician said that he shouldn't really need to be rocked to sleep at his age. But we live with my boyfriends mom. So whenever we try to let him cry it out, she goes in without permission and rocks him to bed. She says we are mean parents for letting him figure it out in his own. She's making parenting really difficult for us. Any advice?