Yes, if you marry in Mexico (and you are a USA citizen), then you should be able to petition for him as your husband (for an immigrant visa / Green Card). All this is assuming that he is not inadmissible to USA.

Has he been to USA before? Has he ever overstayed his visa or came to US illegaly? Was he ever under deportation/removal hearings in Immigration court, was he ever deported from USA, or was here ever caught at the US-Mexican border and sent back?

These are a few major things that you have to keep in mind.

If you'd like to schedule a consultation or need help, please let me know.

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Assuming all other requirements are met, you may apply for him as a K-1 fiancee without getting married. Detailed information can be found here:

http://www.msclaw.com/U_S_Fianc_e_Visa.html

If you do marry him in Mexico, then you would be applying for the K-3 spouse visa. Detailed information on that process can be found here:

http://www.msclaw.com/K_3_Spousal_Visa.html