The answers in the question, your little boy isn't crying for nothing he's crying because he's learnt that you respond to him when he cries, Children at this age get very clingy anyway it's their way of keeping themselves safe being with a familier adult who loves them. why not try doing little activities with him where you can get up when he is playing and go somewhere else in the room where he can still see you, and returning to him a couple of minutes later and engaging him in play again, this way he will learn to become more secure knowing that when you do leave him that you will return shortly.