I am USA citizen but my Mexican does not have my surname. He is still living in Mexico. How do I approach this problem as I would like to petition for

You have to prove that your Mexican son is your son through birth certificate, baptismal certificate, schooling records, or other evidence. A list is on the instructions to the I-130 Petition for Alien Relative form. If unable to provide convincing evidence, you can offer to take a DNA test. In addition, if you are the father (not the mother), you will also have to show proof of parental care of your son prior to his reaching the age of 18 years. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.