Baby Sad When Mommy Away

My wife Allie accompanied her mother to China for five days. To our surprise, my son seemed knowing his mom was to be out of town for more than one or two days. He cried loudly and sadly at the hour when he's supposed to be in mommy's arms, not even my mother who he spent most of the time with could comfort him. A few weeks ago my wife had a business trip and was not home for one night, but he did not react so dramatically.