Let your son know that you will be waiting for him when he finishes. Don't linger or give any indication that there is an alternative to going to school alone. Make sure you solve this problem now and don't let it build. Some kids are so nasty to other kids. This depends on the age and the nature of the nastiness.
Some kids are overly sensitive.
Some lack experience in social situations. You may need to intervene with the school if this teasing is persistent and particularly offensive. If you do intervene, begin by trying to learn more and gather information rather than telling the school what to do. Try to avoid giving your child the feeling that you are their savior and going to fight their battles for them. If they are very young, you probably should say little to nothing.