Hi inh and welcome to ilw,
We did have someone here with similar problems a few months back.
If I remember, I think we suggested that maybe the father registered his son at the Consulate in Mexico and took the paper to the border to be able to cross. Basically a child of a USC born abroad kind of thing. However years ago, the routine to do this was different to today's procedures.
May I suggest you ask your parents where did they register you in said country and ask if they needed to register you at the consulate or somewhere before crossing the border.
Also if I remember rightly, the parents may have had to register you here in the US with the certificate or whatever papers they were given.
If I find the thread, I will post the link here for you. It gave some good suggestions.
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