How can you stop them from continuing crying?

My girlfriends little boy is 13 months and every time she leaves the room (even still viewable by him) he cries hysterically and wont stop until she picks him up. He is very familiar with me and my house. He has been around me almost daily since he was born. He does the same if anything does not go exaclty his way even if he is the one that does it. At first it looked the clic case of being spoiled, but it has gotten so bad it seems like more. Any suggestions of what it is and how to stop this behavior.