I suggest you first contact the school counselor and advise him or her of what is going on. Have your son present. Discuss how the school proposes to handle the situation. Ask for a copy of the school's policy on sexual harassment. (The school has a legal obligation to have one.) The policy should outline the process for handling complaints. Follow whatever the process is.
I expect your son is too beaten down at the present time to do this, but his first legal step is to calmly tell the children who are teasing him to "Stop calling me . . . " Of course they will not. He should keep track of who the perpetrator is, when and where, and exactly what is said. Then the very next time it happens he should report to the counselor. The counselor or assistant principal in charge of discipline should handle it from there. In place of his telling the harasser to stop by himself, your son could tell him or her in front of the counselor. I have even read notes from the victim to the student who is doing the taunting. The school should then follow its discipline process, which should include filing harassment charges if nothing else works.
If the school doesn't do anything about this situation, I encourage you to file harassment charges yourself with the school resource officer or the local police. Take in as much detail as possible. Your son has a right to a safe and non-hostile educational environment and the school has the obligation to provide it. He's very lucky to have a mom who cares.
You may also want to check on what your son's school is doing in the way of anti-violence and anti-bullying programs.