What is your son upset about? If he is being teased, the teasing may be unfair and cruel so he may have reason to cry. The issue would be the teasing and the person doing it. If he is crying because he is frustrated or angry, he may need some help in learning how to express his anger and frustration calmly and assertively. His school counselor may be able to work with him on this or give you some assistance.
When he cries, who does the teasing? Family members? Classmates? If it's family members, then the work must begin in the family to respect each other and work together to create a safe climate where emotions can be expressed.