My 13 month old cries whenever anyone laughs?

In the last few months, my son cries whenever ANYONE laughs - me, my husband, the daycare providers, even if a character on tv is laughing. The only exception is that if he's laughing first, he's fine if you start laughing. He looks terrified: he gets really stiff and rigid and just looks petrified. Has anyone else experienced this? Is th ere anything that I can do to help my baby?