For the most part, this is not an immigration question.
I'll only answer the part that relates to US Immigration and Citizenship law.
1. What makes you so sure that he'll have a visa? If he's from Mexico and isn't in the US legally, and entered before he was 16 ... he can apply for deferred action ... not a visa
2. A child born in Mexico to a US Citizen is a DUAL CITIZEN of both the US and Mexico.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
Whether the child is a US citizen if born in Mexico depends on whether you meet the requirements to transmit US citizenship: Been physically present for 5 years in the US prior to the birth, with two of those years being after the age of 14. My colleagues are assuming that you meet these requirements, but there are no facts in your posting to make that determination.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.