you should meet with the doctor once because it couldd be anything for example a paon in its stomach

You can divert his attention and be more involved with him in play or a activities. It will help you to understand the cause .

Try to divert the mind and change the mood of the baby.. play music or rhymes ..

.This is a very tender an age where his habit formation will take some time ,You can keep your child distracted and also 13 month old behaves the way you are responding to him . You can get him involved with some role play or ensure you get your parents or inlaws involved as a helping hand