Infants and young toddlers feel their emotions very strongly and have little self-control.  This means that he is still learning how to express and cope with strong emotions.  Remember all those emotions a child might be feeling when parents leave- sadness, anger, and fear?  A child who cries when Mom or Dad comes back may be feeling overwhelmed with those same emotions all over again. 

You can help by empathizing and labeling how the child might be feeling to show that you understand those strong emotions.  This will help him to learn that he can depend on you to help him recover when he is feeling overwhelmed.  Eventually, this will also help him to have repeated positive experiences with reuniting after a period of separation.