My ex boyfriend lives in mexico but he doesn't have a social security card

You would need to contact the Mexican Consulate for help. They have an attorney on staff dealing with cross-border child support.

http://www.sre.gob.mx/index.php/representaciones/consulados-de-mexico-en-el-exterior

Hat person will want to confirm the legitimacy of your claim, so bring some documentation (ID, marriage certificate, birth certificate, divorce decree, order of child support, whatever you have).