Directly from the US Department of State Website Regarding Travel to Mexico:
"Since March 1, 2010, all U.S. citizens – including children – have been required to present a valid passport or passport card for travel into Mexico. While documents are not routinely checked along the land border, Mexican authorities at immigration checkpoints approximately 20 to 30 kilometers from the border with the U.S. will often conduct vehicle and document inspections and will require valid travel documents and an entry permit or Forma Migratoria Multiple (FMM). All U.S. citizens entering by land and traveling farther than 20 kilometers into Mexico should stop at an immigration checkpoint to obtain an FMM, even if not explicitly directed to do so by Mexican officials. Beyond the 20-30 kilometer border zone, all non-Mexican citizens must have valid immigration documents (FMM, FM2, FM3 or FME) regardless of the original place of entry. Failure to present an FMM when checking in for an international flight departing Mexico can result in delays or missed flights as airlines may insist that a valid FMM be obtained from Mexican immigration authorities (Instituto Nacional de Migración, INM) before issuing a boarding pass."
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If he doesn't have a US Passport, does he have any of the other forms of ID that satisfy the Western Hemisphere Travel Initiative (WHTI) that went into effect on June 1, 2009? These are:
"U.S. Passport – This is an internationally recognized travel document that verifies a person’s identity and nationality. It is accepted for travel by air, land and sea.
U.S. Passport Card – This is a new, limited-use travel document that fits in your wallet and costs less than a U.S. Passport. It is only valid for travel by land and sea.
Enhanced Driver’s License (EDL) – Several states and Canadian provinces are issuing this driver’s license or identification document that denotes identity and citizenship. It is specifically designed for cross-border travel into the U.S. by land or sea.
Trusted Traveler Program Cards – Global Entry, NEXUS, SENTRI or FAST enrollment cards can speed your entry into the U.S. and are issued only to pre-approved, low-risk travelers. The cards are valid for use at land or sea; the NEXUS card can be used in airports with a NEXUS kiosk."
JANNA I. JAMIL
SAN DIEGO IMMIGRATION ATTORNEY
Kersey and Sabawi, PC
The answer above is provided for informational use only. One should not act or refrain to act solely based on the information provided. No attorney/client relationship is created unless an Agreement is signed by the attorney and the client.